Put a large pan over medium heat.
Slice the onions into very thin strips—the thinner the better so they caramelize faster! Melt 1 tablespoon (15 g) of vegan butter in the pan then add the onions. Sprinkle them with a dash of baking soda and stir. This raises the pH level of the onions, which makes the sugars caramelize faster. Do not use more than ⅛ teaspoon of baking soda or your onions will turn to mush. Cook for 2 to 3 minutes, stirring occasionally.
Make the cheddar cheese. Add the cashews, water, tapioca flour, chipotles, garlic, yeast, agave, vinegar, salt, chili powder and red pepper flakes to the blender and blend on high until smooth and creamy, about 3 minutes.
Add the brown sugar to the onions. Cook until softened and brown, about 7 to 8 minutes. Add the balsamic and then reduce heat to medium-low. Cook for another 2 minutes, moving the larger pieces to the center of the pan. Taste and then add a tiny dash of salt, pepper and more balsamic if needed.
Put a small saucepan over medium-high heat for 1 minute. Pour the cheese sauce from the blender to the hot pan. Stir the cheese, scraping the sides and bottom of pan, until the lumps are gone and the cheese forms a gooey mass in the center of the pan, about 3 to 5 minutes.
Transfer the caramelized onions to a bowl. Place the pan back over medium heat. Thinly slice the tomato.
Build the sandwiches with 3 tablespoons (45 g) of chipotle cheese, 1 to 2 tomato slices and a large spoonful of onions. Spread vegan butter on the outsides of the sandwich. Cook until golden brown and slightly crisp, about 2 to 3 minutes on each side. Serve with kettle chips and a pickle!