Melt Hiland Butter over medium heat in a 9 or 10 inch skillet. Add onions and saute for 2-3 minutes. Add mushrooms and saute until soft. Remove onions and mushrooms and place in bowl.
Wipe skillet clean with a paper towel. Spray well with non-stick spray.
Pour lightly beaten eggs into skillet so that the whole skillet is covered. Cover the skillet. Cook on low flame.
After a minute or two, when egg begins to set, gently lift the edges of the egg with a spatula so that the uncooked egg in the middle runs to the edges of the pan and cooks. Cover again. If needed, repeat the process of running uncooked egg out of the middle of the omelet.
When the egg has started to set, place the onions and mushrooms on half of the eggs. Sprinkle the Hiland Cheddar Cheese on top of the onions and mushrooms. Carefully fold the other half of the omelet over the filling. Cover. Cook on low for another minute or two, just until the cheese is melted.