Heat a medium nonstick skillet over medium heat. Remove stems from mushrooms; discard stems. Coat pan with cooking spray. Add mushroom caps to pan, stem sides up. Pour 1/2 teaspoon lemon juice into each cap (A). Sprinkle evenly with dill and pepper. Cook for 6 minutes or until mushrooms are browned. Turn mushrooms over; cook 1 minute. Remove from pan; cover and keep warm.
Wipe pan with a paper towel. Melt butter in pan over medium heat. Add egg substitute; allow to set for about 30 seconds. Gently scrape cooked part of egg substitute to center of pan with a rubber spatula (B); continue gently scraping occasionally until egg substitute is set (about 3 minutes). Remove from heat.
Arrange muffin halves, cut sides up, on a baking sheet. Top each muffin half with 1 Canadian bacon slice, about 1/3 cup scrambled egg substitute, and 1 mushroom cap, stem side down. Gently press mushroom cap down. Top each mushroom cap with 1 cheese slice. Broil 1 1/2 minutes or until cheese melts. Sprinkle with green onions, and serve immediately.