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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.