In a mixing bowl, combine the ricotta cheese, basil pesto, grated Parmesan cheese, garlic powder, red pepper flakes (if using), salt, and pepper. Mix until smooth and well-blended.
Brush the portobello mushrooms with olive oil and place them cap-side up on a baking sheet.
Generously fill each mushroom cap with the ricotta-pesto mixture, mounding it slightly.
Bake the stuffed mushrooms in the preheated oven for 20-25 minutes, or until they are tender and the tops are golden and bubbly.
Remove from the oven and allow to cool slightly before transferring to a serving plate.
Garnish with fresh basil leaves for a vibrant touch.