Heat up oven to 400F / 204C. Toast flat breads or pitas on a sheet pan for 4 to 5 minutes.
Heat up a skillet or saute pan over medium heat. Add oil and then red onions to heated oil. Saute for ~2 minutes.
Add chicken with cumin, paprika and oregano. Saute for another 2 minutes, just to warm chicken up.
Add red wine vinegar and saute for another 30 seconds and remove from heat. Season to taste with some salt and pepper.
Assemble wraps with chicken, tzatziki, tomatoes, romaine and a squeeze of fresh lemon juice. Drizzle remainder of tzatziki over tomatoes and romaine that cannot fit into wraps and enjoy as a side salad!