Preheat broiler. In a bowl, beat together eggs, milk and seasonings with a fork. In a 10 inch (25 cm) non stick oven proof skillet, melt butter over medium high heat. Pour in egg mixture. As eggs begin to set at edges, use a spatula to gently push cooked portions to the center, tilting skillet to allow uncooked egg to flow into empty spaces. When eggs are almost set on surface but still look moist, spoon salsa on top and garnish with avocado slices sprinkled with lemon juice. Top with cheese. Place skillet under broiler, until cheese is just melted, about 30 seconds. Cut omelet into wedges and serve.