Prepare the bell peppers, onions, carrots, and broccoli by slicing into thin strips of similar size and length for even cooking. Set aside. Reserve the diced cucumber and fresh cilantro for garnish after cooking.
Combine all ingredients for the sauce in a small bowl or measuring cup. Set aside.
Heat a large skillet over high heat. Once the skillet is heated, add the oil and coat the bottom of the pan. Add the sliced veggies and cook on high heat, stirring or flipping occasionally, until edges begin to brown. Leave them at a tender-crisp doneness to avoid overcooking. Remove from heat and transfer to a mixing bowl to hold warm.
Remove the marinated skirt steak from the bag and place it in the center of the pan. Cook about two minutes on one side, then use tongs to flip. Cook about two minutes more, then remove from the skillet. Use a meat thermometer to check internal temp of 145 degrees F. Allow to rest for about five minutes before slicing into thin strips, cutting against the grain. Set aside
Reduce heat to medium-high and add the sauce to the skillet. Deglaze the bottom of the pan, then add the cooked veggies and sliced skirt steak back into the skillet. Stir to combine, then remove from heat.
To serve, place cooked rice in the bottom of a bowl. Top with the skirt steak stir fry, then garnish with diced cucumber, fresh cilantro, and sesame seeds (if using). Serve immediately.