Vegan Patty Melt

Name: Vegan Patty Melt
Cuisine: American
Ingredients: 2 slices Texas Toast (vegan); 2 slices vegan cheese (Swiss, Provolone etc); 2 4 oz. plant-based burger patties (like Beyond, Impossible); 1 cup Vegan Mayo; 2 tsp yellow mustard; 2 tsp vinegar; 2 Tbsp diced chipotles in adobo; 1/4 tsp onion powder; 1/4 tsp garlic powder; Salt & pepper to taste; 3 lbs. Yellow or Brown Onions (diced); 2 Tbsp vegan butter; 2 Tbsp neutral oil; Salt to taste; Sugar to taste; 1 Tbsp Balsamic vinegar (optional); 1/4 - 1/2 cup vegan beef broth (optional)