Preheat your oven to 375°F (190°C). Line a muffin tin with parchment paper or grease it lightly.

In a medium bowl, combine the flour and salt for the crust. Add in the cubed butter and mix until it resembles coarse crumbs. Gradually add the cold water, mixing until the dough holds together.

Roll out the dough on a floured surface to about 1/8 inch thick. Cut out circles using a biscuit cutter or a glass, large enough to fit into your muffin tin cups. Press the circles into the cups with your fingers, creating little crusts.

In a skillet, heat olive oil over medium heat. Add the onion and garlic, sautéing until translucent, about 3 minutes.

Stir in the eggplant and bell pepper, cooking for an additional 5-7 minutes until softened. Add the zucchini, yellow squash, diced tomatoes, thyme, and basil. Season with salt and pepper. Cook until the vegetables are tender and the mixture thickens slightly, about 8 minutes.

Spoon the ratatouille filling into each tartlet crust, filling them generously but not overflowing.

Bake the tartlets in the preheated oven for about 25-30 minutes or until the crust is golden brown.

Let the tartlets cool slightly before garnishing with fresh basil and Parmesan cheese if desired. Serve warm or at room temperature.