Slice red peppers in half. Place on a parchment lined baking sheet. Drizzle olive oil on peppers. Roast at 450°F for 15 minutes, or until charred.
Let cool. Peel off most of the skin, leaving a little on for flavor.
Heat 2 tablespoons olive oil in a pan on medium low heat. Add sliced onions. Sprinkle salt on onions.
Stir the onions every few minutes or so. Cook about 35-40 minutes until they are well-browned. Add a couple of splashes of water if they start to stick. Add balsamic vinegar and cook until it's evaporated.
Warm focaccia in oven or microwave. Spread vegan mayo on half the slices.
Layer roasted red pepper and caramelized onion on top. Top with other slices of focaccia.
Notes
When making caramelized onion, slow and steady wins the race. For caramelized onions, you need some patience. It takes a good 40 minutes to get nicely browned onions.If they start to stick add a little water - like a quarter of a cup or so.Balsamic vinegar adds a wonderful flavor. I always add a couple of splashes near the end of the cooking time.I recommend making the sandwiches right before serving.The focaccia with sun-dried tomato and black olives will keep for a few days. I had mine as long as six days and it still tasted good. Just warm it up and you're good to go.