Heat a drizzle of olive oil in a large non-stick frying pan over medium-high heat. Add mushrooms and cook until golden brown. Remove a large spoonful of mushrooms and set these aside for mixing into the eggs. To the remaining mushrooms- stir in shallot and garlic and cook for 1 minute until fragrant. Add asparagus and cook for 1-2minutes until tender crisp. Stir through peas until just cooked. Remove pan from heat, set filling aside and keep warm.Whisk eggs in a large bowl, 2 at a time. Add some of the reserved cooked mushrooms. Heat a drizzle of olive oil and 1 tsp butter in a frying pan over medium-high heat. Pour in the egg and mushroom mixture and swirl to coat base. Season the eggs. Cook omelette for 1-2 minutes or until just set. Add a quarter of the warm filling to the one side. Crumble over a quarter of the feta and give everything a little squeeze of lemon juice. Close the omelette and carefully slide onto a plate. Repeat with remaining eggs to make 4 omelettes.