Pizza actually can be somewhat healthy, so long as you're using a small amount of cheese, lots of veggies, and spices. These guilt-free bad boys are perfect for an adults-only or family holiday gathering.
Preheat the oven to 400°. Line a 9x13" sheet pan with parchment paper or foil.
Heat the oil in a large pan set over medium heat. Add the onion and cook, stirring often, until softened—about 5 minutes. (You may need to adjust the heat to prevent burning.) Stir in the zucchini, squash, and salt, and cook an additional 5 minutes, stirring frequently. Add the garlic and oregano, and cook one more minute. Remove pan from the heat.
Spread marinara sauce evenly over bread slices, then sprinkle with cheese. Top each slice with some of the squash mixture. Transfer pan to the oven, and bake for 10 minutes—or until bread is toasted and cheese has melted.
Garnish with fresh basil before serving.