Heat a pan over medium heat and spray pan with cooking spray. Place one half of each English muffin, or 2 pieces of bread if using, on the pan. Top with the roasted veggies, then the cheese and then the sliced avocado. Add another light sprinkle of sea salt, and then top each muffin half or piece of bread with the other half or piece of bread. Cook for about 3 minutes, then using a spatula, flip the sandwich over and cook on the other side for about 3 minutes, or until the cheese begins to melt and the bread is toasted.