Skirt steak is well-marbled and should be cook medium rare; any more, it will toughen.
Heat a grill or grill pan to high. In a large bowl or zip-top bag, combine lemon juice, 1/2 teaspoon thyme, bell peppers, zucchini, and onion; season with salt and pepper and marinate 15 minutes.
Season steak with salt, pepper, and remaining 1/2 teaspoon thyme. Clean and lightly oil hot grill. Cook steak, flipping once, about 12 minutes for medium. Transfer to a cutting board and tent with foil. Let rest 10 minutes before thinly slicing against the grain.
Lightly oil hot grill. Arrange vegetables on grill in an even layer, reserving marinade, and cook until browned and tender, 8 to 10 minutes. Whisk oil and mustard into marinade. Cover and refrigerate half the vegetables and half the dressing for Zesty Pork Sandwiches. Serve remaining vegetables and steak with remaining dressing.
A lemon marinade is a triple threat: It flavors vegetables before grilling, then makes the base for a tangy dressing to use tonight and for Zesty Pork Sandwiches.