Halve the slices of bologna, place them one on top of the other and cut into long strips 1 cm wide. Place sausage strips in a large enough bowl, carefully pick apart and mix with 1 tbsp. oil. Cut cornichons lengthwise into thin slices, then into narrow strips. Halve onions and cut lengthwise into thin strips. Mix mortadella, cornichons and onions. Cut cherry tomatoes in half. Coarsely crumble feta cheese.
Pluck the leaves from 1 bunch of basil, chop coarsely and blend with egg yolk and vinegar in a high bowl, adding the remaining oil drop by drop at the beginning, then in a thin stream and beat until creamy. Season vigorously with salt and season with pepper.
Layer mortadella leaf lettuce, sauce and tomatoes on a large platter. Sprinkle with the remaining basil leaves and feta cheese.