(Start to Finish: 20 Minutes)
Heat olive oil in a large skillet over medium-high heat.
Add the sliced onion and cook for 8 to 10 minutes, stirring frequently, until very tender and golden.
Meanwhile, heat the veggie burger patties according to the package directions.
In a small bowl, whisk together the mayonnaise and mustard.
Spread the mixture onto one side of each toasted ciabatta slice.
Layer the sandwiches by placing spinach on the bread, followed by a warm veggie patty.
Stir the steak sauce into the cooked onions in the skillet, then spoon the onion mixture over the patties to serve.
