Slice the tomatoes into fairly thick slices (about 1/2-inch wide slices). Lay the tomato slices on a large flat serving plate or platter. Sprinkle a little salt on each slice of tomato.
For each tomato slice, cut one very thin slice of red onion. Lay a slice of red onion over each slice of tomato.
Generously drizzle olive oil over the top of each slice of onion so that the tomato is covered in the olive oil as well. Depending on the size of your plate, you may need more olive oil.
Sprinkle parsley on top of each slice of onion. Cover with plastic wrap and let sit in the refrigerator for an hour before serving.
Just before serving, top with crumbled blue cheese.