Stir together the sugar, chili powder, cumin, garlic powder, paprika, 1 1/2 teaspoons salt and 1/2 teaspoon pepper in a small mixing bowl to make the dry rub for the steak.
Put the steak on a parchment-lined baking sheet. Rub both sides generously with the spice rub. Allow the steak to come up to room temperature for about 30 minutes. Meanwhile, prepare the vegetables.
Place a grill pan over medium-high heat and allow the pan to heat for 5 minutes.
Put the bell peppers and onions in a large mixing bowl. Drizzle with the olive oil. Sprinkle generously with salt and pepper. Spread the vegetables out over the hot grill pan. Cook, tossing occasionally, until caramelized and softened, 8 to 10 minutes. Put the grilled vegetables on a serving platter and tent with aluminum foil to keep warm.
To cook the steak, brush the hot grill pan with the vegetable oil. Place the steak on the grill and cook for 3 minutes. Flip the steak over and cook for another 3 minutes. Transfer the steak to a cutting board and allow it to rest for at least 5 minutes.
Slice the steak into thin strips, against the grain of the meat. Layer the steak over the warm vegetables and sprinkle with the cilantro leaves. Serve with shredded iceberg lettuce, sour cream and warm tortillas.
If not using the rub right away, store in an airtight container until the steak is ready to be cooked.