To make the veggie filling: In a large skillet over medium heat, heat 2 teaspoons olive until shimmering. Add onions and a dash of salt. Cook, stirring occasionally, until the onions are softened and turning translucent, about 5 minutes. Add the garlic and a pinch of red pepper flakes, stir, and cook for another 30 seconds. Add the zucchini, yellow squash and bell pepper. Cook, stirring often, until the squash is softened and cooked through but not mushy, about 7 minutes. Remove the pan from heat and squeeze the juice of ½ of a lime over the veggies. Season to taste with salt, stir to combine, then set the pan aside.