In a medium-sized bowl combine the squash and olive oil. Season with salt and pepper to taste. Spread the squash about 2 inches deep in an oven proof casserole dish. Drop ricotta cheese on top by the spoonful. Scatter the cubes of mozzarella cheese over the top. Bake for about 10 minutes in a 375 degree (F) oven, or until the cheese has melted and is bubbly. Remove from the oven, cool for about 5 minutes, then drizzle the pesto over the top and serve.