Fill a large saucepan with about 2 inches of water and add the lemon juice or vinegar and half of the salt; bring to a gentle simmer over high heat.
Crack one egg into a small bowl or mug, then gently slide it into the simmering water; repeat for remaining eggs, lowering the heat as needed to maintain a gentle simmer.
Poach the eggs for about 3 to 4 minutes, until the whites are set but the yolks remain runny; use a slotted spoon to remove the eggs and transfer them to a plate.
Toast the bread slices in a toaster until golden.
Top each toasted slice with a lettuce leaf, a few slices of roma tomato, and one poached egg.
Season each assembled toast with the remaining salt and the black pepper, then serve immediately.