Preheat oven to 400 degrees F.Grease a baking tray with oil, and place the mushroom caps on the tray with the stem side up.Heat a dry pan on the stove top over medium high heat.

Add chopped walnuts to the pan and toss for 5 minutes until they are browned and toasted.Place toasted walnuts and the rest of the pesto ingredients in a food processor and blend until the pesto is completely combined and creamy.

Divide up the pesto among the 4 portobello mushroom caps and pour into each of the caps.Layer with salsa sauce, pepperoni slices, and crushed red pepper flakes, if using.

Bake in the preheated oven for 15-20 minutes.Garnish with extra basil leaves before serving