Preheat oven to 375 degrees.
In a large mixing bowl, add your cup of cooked lentils. Then smash them, I like to use the back of a fork or a potato masher. Smash them pretty well, but so that they still have some texture, you don't want a smooth paste. It should still be chunky.
Then in a small bowl, whisk together the flaxseed and the water. Set aside to thicken for a minute to make a flax egg.
Then to the bowl with the lentils, add the panko, olive oil, liquid smoke, soy sauce, garlic powder, salt and pepper.
Then add in the flax egg. Stir everything together until fully combined. The mixture should hold together easily.
Now form patties from the lentil mixture. It will make 2 large patties or 4 small ones. Divide the mixture and roll into a large ball, then flatten into a patty.
Spray a baking sheet with non stick spray, then place the patties on the baking sheet, and spray the tops of the patties with more non stick spray. Then bake for 20 minutes, flipping the patties after 10 minutes. Or until the patties are firm.
While the patties are baking make the Frisco sauce. In a medium sized mixing bowl, whisk all the sauce ingredients together. Taste, adjust seasonings, set aside.
Once the patties are done, spread one piece of bread with the Frisco sauce, then add sliced onion, then a patty, then vegan cheese if you want. Then more sauce on the other slice of bread. Put the sandwich together.
Heat a pan on medium high, and either add some olive oil to the pan, or spread the top piece of bread with vegan butter. Then place the sandwich buttered-side down in the pan.
Press the sandwich down in the pan, and brown one side for a minute or so. Then spread the other piece of bread that is on top now with vegan butter. Flip and brown that side, smashing down the sandwich.
Then, once the melt is all brown and hot and gooey, remove from the pan, and serve immediately!