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Tomato Portobello Gratin - A fantastic, flavorful side dish or meatless meal.

We’re finally planting our backyard garden this weekend, and I’m already counting the days until our garden tomatoes are ready to eat! (I think it’s one of the best things about summer!)

But in the meantime, we were happy to find some great beef steak tomatoes at our local supermarket – and my husband Jack made this fantastic Tomato Portobello Gratin that we’re sharing with you today.

Tomato Portobello Gratin - A fantastic, flavorful side dish or meatless meal.

This Tomato Portobello Gratin is layer after layer of delicious goodness!  Thick slices of those beef steak tomatoes are layered with halved and grilled Portobello mushrooms (buy the large mushroom caps if you can) and placed on top of baguette slices in a roasting pan.

Tomato Portobello Gratin - A fantastic, flavorful side dish or meatless meal.

Then – the layers are topped with a heavenly breadcrumb, Gruyère Swiss cheese, and olive oil mixture that is so good, I kept sneaking bites of the topping while Jack mixed it up. One thing to note here – while a conventional Swiss cheese can be substituted in the topping, if at all possible, we recommend using Gruyère Swiss because the flavor is much better.

Finally, bake the Tomato Portobello Gratin until golden and crispy on top. Then, once the gratin sets up (about ten minutes after it comes out of the oven), slice and serve alongside your favorite roasted or grilled meat. Or – this Tomato Portobello Gratin also makes a delicious meatless meal.

Tomato Portobello Gratin - A fantastic, flavorful side dish or meatless meal.

Any leftovers you have of this Tomato Portobello Gratin are just as delicious – in fact, we think the flavors were even better the next day!  For best results, reheat in the oven so that luscious, crumb topping stays crispy.

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Tomato Portobello Gratin

  • Prep Time: 15 mins
  • Cook Time: 1 hour
  • Total Time: 1 hour 15 minutes
  • Yield: 4-6 servings


  • 2 ½ pounds beef steak tomatoes
  • 1 pound Portobello mushroom caps
  • ½ cup extra virgin olive oil, plus 2 tablespoons divided
  • 1 French baguette, divided
  • 2 teaspoons fresh thyme, divided
  • 6 ounces Gruyère Swiss cheese


  1. Preheat oven to 375 degrees F and have an 8 quart shallow casserole dish standing by.
  2. Remove stem and core of tomatoes then cut each one into four equal thick slices. Lay slices on a layer of paper towels then cover the tops with more paper towels. Let sit until needed.
  3. Slice mushroom caps in half by standing cap on its side and cutting in half so you have two round slices.
  4. Brush one side with 2 tablespoons of olive oil.
  5. Heat a griddle or a few grill pans to medium high. Then place mushroom slices onto pan(s) oiled side down. Brush the other side with two more tablespoons of olive oil. Grill for 4-5 minutes per side. Remove to a platter and set aside.
  6. Brush the casserole dish with two tablespoons of the olive oil.
  7. Cut about half the bread into slices, enough to cover the bottom of the casserole dish as a single layer.
  8. Layer half the tomato slices on top of the bread and sprinkle with half of the thyme.
  9. Layer all of the cooked mushroom caps over the tomato.
  10. Place the rest of the tomatoes over the mushrooms.
  11. In a food processor, place the remaining bread (about 2 cups but no more than 2 cups) cut into pieces, along with Gruyère cheese cut into pieces and the remaining thyme and process to a medium crumb consistency. Place into a large bowl and add the remaining ¼ cup of olive oil and toss to coat.
  12. Pour this mixture over the top of the casserole and bake uncovered in the center of the oven for 45 minutes or until golden brown.
  13. Let sit for ten minutes, and then serve.

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Tomato Portobello Gratin - A fantastic, flavorful side dish or meatless meal.

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  • Mary wrote:

    This dish looks so amazingly delicious! Tomatoes, grilled mushrooms, cheese and bread…..perfection on your plate! Pinned. Looking forward to making this. Thanks, Jack.

    • Martha wrote:

      Thank you so much Mary! We hope you love the recipe!

  • sue | theviewfromgreatisland wrote:

    This is SO up my alley, I have a thing for baked tomatoes, the flavor get so deep and rich — I’ve got this on the menu list for next week 🙂

  • Dee Frantz wrote:

    Everything you post on your site looks so delicious! I’m thrilled I found your website. I made a few dishes and they were wonderful. Bless you both. Again I thank you. Dee

    • Martha wrote:

      Thank you so much Dee! We’re so glad you found us too! 🙂 Have a great weekend and thanks for taking the time to write to us today!

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