Preheat oven to 450°F.
In a shallow baking dish, mix together the vinegar, Bragg Liquid Aminos or soy sauce, olive oil, and ½ teaspoon of the black pepper with a whisk. Place tofu slices in a single layer in the dish, then turn over to coat on all sides. Allow tofu to marinate for 20 minutes, turning slices occasionally.
Place baking dish in oven for 20 minutes. Flip tofu slices over and continue to bake for an additional 10-20 minutes until crispy and most of the liquid has been absorbed.
Meanwhile, spray a large skillet with oil or cooking spray. Sauté onion and garlic over medium-high heat until onion begins to soften. Add mushrooms and continue cooking until they begin to brown. Add tomato, spinach, thyme, salt, and remaining black pepper. Cook and stir until spinach is wilted and any liquid has evaporated, turning heat to medium low if the vegetables are browning too quickly. Adjust seasoning.
To assemble sandwiches, spread vegan butter on English muffins, if using. Then add 2 tablespoons of the cooked vegetables to the bottom half of each muffin. Top with 2 slices of tofu and evenly distribute the remaining vegetables on top of the tofu slices on all 4 sandwiches. Cover with the other muffin halves and press down to help keep vegetables from spilling out.