Over medium-low heat, melt the vegan butter in a thick-bottom stainless steel or cast iron pan. Add the sliced onions and cook for 10 minutes to soften them. Add a pinch of salt and the sugar, then reduce the heat and continue to cook the onions for 30–60 minutes until they are deep amber in colour and caramelized. Stir them just often enough so they brown but don’t burn.
Finish off with the balsamic vinegar, thyme, and garlic. Cook for an additional 3 minutes and set aside.
Place the vegan mayonnaise, garlic, lemon juice, and sea salt in a blender or food processor.
Blend until smooth. Scrape down the sides and add as much water as needed. Blend and repeat until the desired consistency is reached.
Refrigerate in an airtight container until ready to use. Vegan aioli will last up to seven days in the refrigerator.
Cook the Beyond Burger patty according to the package instructions. 2–3 minutes before the patty is finished cooking, add a slice of Daiya smoked Gouda cheese and allow it to melt.
Toast the ciabatta buns for 2–3 minutes. Top with the Beyond Burger patty followed by the caramelized onions and fresh arugula.
Spread the garlic aioli on the top ciabatta half and enjoy!