Prep everything first: crack the eggs into a bowl and beat them lightly with a fork or whisk until yolk and white are combined and slightly frothy — this adds air for a lighter omelette. Season with a pinch of salt and a few grinds of black pepper. Rinse and spin-dry the spinach (or thaw and squeeze out excess water if using frozen/cooked). Slice the mushrooms and finely chop the shallot/onion if using. Toast the whole-grain bread now so it stays warm and crisp.

Warm the pan: place a nonstick or well-seasoned 8-inch (20 cm) skillet over medium heat for 1 minute. Add 1 tsp olive oil (or butter). Heating the pan first ensures the oil shimmers and coats the surface quickly so the vegetables sear, not steam.

Sauté aromatics and mushrooms: add the chopped shallot/onion (if using) and sauté 30–45 seconds until translucent to build flavor. Add the sliced mushrooms and a pinch of salt; cook 2–3 minutes, stirring occasionally, until the mushrooms release their moisture and develop light golden edges. Browning the mushrooms concentrates their savory (umami) flavor; avoid overcrowding the pan so they can brown rather than steam.

Wilt the spinach: add the fresh spinach to the pan and stir until just wilted, 30–60 seconds. Season lightly with salt and pepper. Spinach cooks very quickly — wilting it concentrates the flavor and removes excess moisture that would make the eggs watery.

Transfer the filling to a warm plate and reduce the heat to low. Wipe the skillet with a paper towel if there's excess moisture or oil left — a relatively dry surface helps the eggs set evenly without sticking.

Cook the eggs: return the skillet to very low–medium-low heat and add a small pat of butter or a few drops of oil to coat. Pour the beaten eggs into the pan and let them sit undisturbed for about 20–30 seconds until the edges begin to set. Using low heat and patience prevents the eggs from browning too quickly and keeps them tender and custardy.

Gently push and tilt the pan: with a silicone spatula, gently pull the set edges toward the center while tilting the pan to let the uncooked egg flow to the edges. Do this a few times over 30–45 seconds to create an even, mostly set surface with a slightly soft center — this technique yields a silky omelette rather than a tough one.

Add the filling and cheese: when the eggs are mostly set but still slightly moist on top, scatter the sautéed spinach and mushrooms over one half of the omelette. Sprinkle 1 tbsp grated cheese over the filling (optional). The residual heat will melt the cheese and finish cooking the eggs without overcooking.

Fold and finish: use the spatula to fold the empty half of the omelette over the filling, creating a clean half-moon. Let it cook for another 15–30 seconds to meld the filling and finish setting the center. For an extra glossy finish, tip the omelette onto a plate seam-side down and dot with 1 tsp butter to melt.

Plate: place the toasted whole-grain bread alongside the omelette. Season the omelette with a final pinch of flaky salt and a crack of black pepper. Serve immediately while warm and tender.