Get all of your ingredients ready: chop your veggies, shred your cheese, etc.
In a small bowl, whisk the eggs with a fork.
Heat a small nonstick skillet over medium-high heat. When the pan is hot, add the butter and reduce the heat to medium. The butter will melt, sizzle, and get foamy. At this point, stir the butter for a minute or two, until it's browned. It won't take long since it's a small amount of butter, so keep a close eye on the pan to prevent burning. When the butter has browned, it will give off a bold and nutty aroma. You'll also notice some little brown bits in the pan—that's flavor! So keep them.
Add the eggs to the pan and whirl them around once to cover the entire bottom of the pan. Sprinkle with some salt and pepper.
Once the edges of the egg start folding up and look cooked, carefully glide a silicone spatula around the edges.
Sprinkle the cheese on one side of the omelette, then add the spinach, tomatoes, onions, parsley, and sardine fillets on top of the cheese.
Using the spatula, carefully fold the other side of the omelette over, and carefully slide it onto a plate. Hit it with some extra cracked black pepper and some chopped fresh parsley.