Preheat the oven to 375°F and line a baking sheet with parchment paper.

Halve the zucchini lengthwise and use a spoon to scoop out the centers, leaving a thin border so the shells hold their shape.

Lightly season the cut sides of the zucchini with salt and pepper and arrange them cut side up on the prepared baking sheet.

Heat the olive oil in a large skillet over medium heat.

Add the garlic and onion and sauté for 2 minutes until softened and fragrant.

Add the chopped mushrooms and cook for 3 to 4 minutes, stirring occasionally, until softened and lightly browned.

Stir in the chopped spinach and cook for 1 to 2 minutes until wilted and bright green, then remove the skillet from the heat.

Add the ricotta, Parmesan, red pepper flakes, 1/2 teaspoon salt, and 1/4 teaspoon pepper to the skillet and stir until the mixture is creamy and well combined.

Spoon the ricotta mixture evenly into the zucchini boats, pressing it gently so it fills the cavities without spilling over.

Bake the stuffed zucchini for 20 to 25 minutes, until the zucchini is tender when pierced with the tip of a knife and the tops are lightly golden.

Remove from the oven, garnish with fresh basil, and serve warm.