Prep the lemon and arugula: zest the lemon first (zest holds aromatic oils) then cut it in half and squeeze 2 tablespoons of juice into a small bowl, catching any seeds. Place the arugula in a medium bowl. Toss the arugula with 1 teaspoon of the lemon juice and a small pinch of salt — this lightly dresses the greens so they stay vibrant on the toast instead of wilting under the hot bread.

Toast the bread: heat a nonstick skillet over medium or preheat a toaster. Lightly brush one side of each slice with the optional butter or a few drops of olive oil. Toast the bread until golden and crisp (about 2–3 minutes per side in a skillet, less in a toaster). Why: toasting creates a sturdy, crunchy base that prevents sogginess from the avocado while adding caramelized flavor and color.

Prepare the avocado: halve the avocado, remove the pit, and scoop the flesh into a bowl. Add the remaining lemon juice (this protects color and adds brightness) plus a generous pinch of sea salt and 1/4 tsp black pepper. Mash with a fork until mostly smooth but still slightly chunky — you want creamy spreadability with a little texture to contrast the smoked salmon.

Assemble the base: while the bread is still warm, spread about half of the mashed avocado on two slices and the other half on the remaining two slices. Spreading on warm bread helps the avocado meld slightly with the toast for a silkier mouthfeel; spreading on cold bread would remain too firm.

Layer the smoked salmon: arrange 50 g of smoked salmon over the avocado on each serving (about 50 g per person, arranged in delicate folds). Folding the salmon rather than laying it flat traps air and creates a restaurant-style look and varied texture — and distributes the smoky flavor evenly across bites.

Finish with greens and seasoning: top each toast with half the dressed arugula. Drizzle each toast with 1/2 tablespoon extra-virgin olive oil (total 1 tbsp for recipe), sprinkle a light pinch of lemon zest over the salmon for aromatic lift, and finish with a grind of fresh black pepper. The oil adds richness, the zest lifts the smoked flavors, and fresh pepper provides a sharp counterpoint.

Plate and serve immediately: serve the toasts warm for the best contrast of textures. If making ahead for more than 10 minutes, keep avocado and toast separate and assemble just before serving to avoid sogginess.