In a large bowl, combine the ground turkey, bread crumbs, diced onion, chopped parsley, egg, garlic powder, salt, and black pepper; mix gently with your hands or a spoon until just combined, taking care not to overwork the meat.
Divide the mixture into 12 equal portions, roll each into a ball, and press each ball flat to form slider-sized patties about the thickness you prefer.
Heat the olive oil in a large skillet over medium-high heat until hot.
Place the patties in the skillet (work in batches if needed) and cook 2–4 minutes per side, until an instant-read thermometer inserted to the center reads 165°F (74°C).
If using cheese, top 6 of the patties with a slice during the last minute of cooking and cover the pan briefly to melt.
While patties cook, mix the mayonnaise and ranch seasoning together in a small bowl.
To assemble each slider: spread the ranch mayo on the inside top of each roll, place the bottom roll, add lettuce, a tomato slice, a few rings of red onion, the turkey patty (with cheese if used), and finish with the dressed top roll.