For the Tex-Mex chili rub: Mix together the paprika, brown sugar, chili powder, cumin, chipotle chile powder, garlic powder and 1 teaspoon salt in a small bowl. Toss the flank steak in a container with the olive oil and lime juice, then rub with the spice mixture. Refrigerate, covered, for at least 30 minutes and up to 2 hours before cooking.
For the squash and zucchini: Heat 1 tablespoon of the olive oil in a large skillet over medium-high heat. Add the zucchini and a pinch of salt and cook until the zucchini is light brown and tender, about 3 minutes. Transfer to a plate. Repeat with the remaining 1 tablespoon olive oil, yellow squash and a pinch of salt, then transfer the cooked squash to the plate. Add the scallions to the skillet and cook, stirring once or twice, until lightly charred. Add the tomato, jalapeno and the cooked zucchini and squash and stir to combine. Add the cilantro and season with salt. Set aside.
Prepare a grill for medium-high heat and brush the grill grates with oil.
Grill the flank steak until marked, 4 to 5 minutes per side for medium rare. Transfer to a cutting board and thinly slice.
Serve with the warm tortillas and toppings.
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