Heat a grill pan over high heat. Brush portobellos with 1 1/2 tablespoons of the oil. Stir together cumin and chile powder in a small bowl. Sprinkle half of cumin mixture over portobellos, pressing to adhere. Grill portobellos until lightly charred and tender, 5 to 6 minutes per side. Remove from heat.
Heat remaining 1 1/2 tablespoons oil in a large skillet over high heat. Add onion, bell peppers, and poblano; sprinkle with remaining cumin mixture and 1/2 teaspoon of the salt. Cook, stirring often, until lightly charred and tender, 4 to 6 minutes.
Heat tortillas according to package directions. Thinly slice portobellos; sprinkle with remaining 1/2 teaspoon salt. Divide portobellos evenly among tortillas. Top evenly with onion mixture. Serve with lime wedges and hot sauce, if desired.