Crack eggs directly into small mixing bowl. Add milk, salt, and pepper to bowl, then vigorously whisk ingredients until fully combined. Mixture should be pale in color, with no streaks remaining. Set bowl aside.
Heat skillet over medium-low heat. When pan is warm, add oil and swirl pan to distribute oil evenly across surface. Continue heating skillet until oil is hot and shimmery.
When oil is hot, slowly pour egg mixture into skillet. Gently tilt and swirl pan to spread egg mixture out evenly, covering entire surface of pan. Return skillet to heat.
Cook egg mixture 2 to 3 minutes, or until eggs are not quite fully set. Mixture should be thick and no longer runny, but should still look slightly wet (shiny).
Working quickly, cover one half of omelette with chicken, cheese, spinach, and tomatoes, distributing ingredients evenly across eggs.
Gently shake skillet to loosen bottom of omelette from pan, or run silicone spatula under edges of omelette to loosen. Use spatula to carefully lift edge of plain omelette half, then fold plain half over to cover filling. Gently press on top of folded omelette with spatula to make top half stick to bottom half.
Once folded, immediately slide omelette out of skillet and onto serving plate. Serve warm with desired sides and/or toppings.
Recipe Notes
Tomatoes: Drain as much excess liquid from the tomatoes as you can, to keep your omelette filling from being wet or runny.
Make it Dairy Free: Use dairy-free cheese and dairy-free milk.
Make it Whole30/Paleo: Omit the cheese entirely. Replace the milk with unsweetened almond milk or unsweetened coconut milk.