Prepare barbecue (medium heat). Puree first 5 ingredients in processor. Season marinade with salt and pepper.
Place chicken breast halves in 13x9x2-inch glass baking dish. Pour 1/3 cup marinade over; turn to coat. Arrange poblano chiles, bell peppers, and onions on large rimmed baking sheet. Pour ½ cup marinade over; turn to coat. Sprinkle chicken breasts and vegetables with salt and pepper. Reserve remaining marinade.
Grill chicken until cooked through, about 7 minutes per side. Grill vegetables until tender, turning frequently, about 15 minutes for onions and 12 minutes for poblanos and bell peppers. Grill tortillas until charred, about 1 minute per side.
Transfer chicken to work surface; slice crosswise into strips. Fill tortillas with chicken and vegetables; drizzle with reserved marinade. Serve with optional toppings.