Halve the avocado lengthwise, remove the pit, and scoop the flesh into a small bowl. Mash with a fork until mostly smooth with a few chunks remaining. Stir in the lemon juice, 1/8 tsp kosher salt and 1/8 tsp black pepper. Set aside.

Prep produce and condiments

Thinly slice the tomato into 4–6 slices. Slice the red onion paper-thin and separate lettuce leaves. In a small bowl mix the mayonnaise and Dijon mustard until smooth.

Toast the bread (optional)

If you prefer a warm or crisp sandwich, brush one side of each bread slice lightly with the olive oil. Heat a skillet over medium heat and toast the bread oil-side down 1–2 minutes per side until golden, or toast in a toaster. Remove and place on a clean work surface.

Assemble first layer

Spread half of the mayo-Dijon mixture evenly on one slice of bread (non-toasted side if toasted). On top of the spread, arrange 4 slices of turkey breast evenly.

Add cheese and greens

Place one slice of cheese over the turkey, then top with the lettuce leaf. If using toasted bread and you prefer melted cheese, place the open sandwich under a broiler or in the skillet briefly until the cheese softens, 30–45 seconds—watch closely.

Add tomato and onion

Layer the tomato slices evenly, season the tomato lightly with the remaining pinch of salt and pepper, then add a few thin rings of red onion on top.

Spread avocado

Spread half of the mashed avocado over the remaining slice of bread (or dollop on top of tomato). This creates a creamy barrier that prevents the bread from getting soggy.

Finish and serve

Close the sandwich with the avocado-covered slice of bread, press gently to compact. Repeat assembly to make the second sandwich using the remaining ingredients. If desired, cut each sandwich in half on the diagonal. Serve immediately.