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Preheat the oven to 425°F.
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Line a baking sheet with parchment paper, and add the portabello mushrooms tops down onto the baking sheet, with the insides facing up.
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Pulse the basil, walnuts, parmesan, and olive oil into a pesto.
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Salt to taste and spread evenly onto mushroom bottoms.
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Layer with chopped tomatoes, and shredded mozzarella and bake for 20 minutes.
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Adjust for salt, as needed and enjoy with red pepper flakes.