Heat 1 tsp olive oil in a small skillet over medium heat. Add mushrooms, shallot, garlic, tomatoes and spinach. Sauté until all veggies are tender, about 5 minutes, stirring occasionally. Once cooked, transfer veggies to a bowl and set aside.
In the same skillet, heat 1 tsp olive oil over low heat.
In a bowl, whisk the eggs. Pour into skillet. Allow egg to cook for about 2 minutes, without stirring. Add the cooked veggies to one side of the egg. Sprinkle the feta cheese on top. Using a spatula, flip the other side of the omelet over the side with veggies, to create a half moon omelet shape. Cook for another 2 minutes, until the eggs are fully cooked.
While the omelet finishes cooking, toast 1 slice of whole wheat bread. Once toasted, top with half of an avocado, using the back of a fork to mash. Top with salt and pepper to taste.