Add a can of coconut milk, water, nutritional yeast, tapioca starch, salt, lemon juice, mustard, paprika, garlic powder, and turmeric, then give it a stir with a whisk until all of the starch is dissolved. (Don't turn on the heat until the starch is fully dissolved.)
Turn the stove to medium heat and stir frequently until your cheese sauce thickens and starts to boil. Then turn down the heat to low and stir constantly for about 30 more seconds before taking off the heat.
Make the steak and pepper filling
Peel and slice an onion into long thin slices and saute it in a tablespoon of oil over medium-low heat for about 5 minutes.
While the onion is cooking, wash and cut bell peppers into long, thin strips, then add them to the skillet with the onion. Sprinkle the veggies with a little salt, then saute for a few more minutes until just barely tender.
Toss in a thawed package of frozen vegan beef strips (or thinly cut strips of any beef-style seitan product) and cook for about 3-5 more minutes until the faux beef is heated through.
Remove the steak and pepper mixture from the skillet and add a tablespoon of oil to the bottom of the pan. Cut your buns in half and grill them on the skillet for about 2-3 minutes each until they turn golden brown.
Assemble your vegan cheese steaks. Add the pepper, onion, and vegan steak strips to the bun and drizzle vegan cheese sauce over the top. Serve hot.
Notes
Tips for the cheese sauce-
Use full-fat coconut milk for the creamiest cheese sauce, but you can use light coconut milk for a lower-fat option.
Add the tapioca starch to cold coconut milk. If you try to add this after you start heating the sauce, it will clump and not dissolve.
Tapioca starch will give you the best results for a cheesy texture that won't become gloppy when it starts to cool down.
Tips for the cheesesteak-
Toast your bread before assembling your cheese steak. It will make it crispier and taste more authentic.