Give chicken a rest and try a turkey cutlet for this one-pan dinner. The tomatoes release liquid as they cook, steaming the cauliflower without added water.
In a large skillet, heat oil over medium-high. Season turkey with salt and pepper; cook until opaque throughout, 2 to 4 minutes per side. Transfer to a plate, and set aside (reserve skillet).
Reduce heat to medium; add cauliflower and onion to skillet. Cook, tossing occasionally, until onion is golden, 7 to 10 minutes. Add tomatoes; cover skillet, and cook until cauliflower is tender, 5 to 7 minutes. Return turkey to skillet; cover, and cook until turkey is heated through, 2 minutes.
Transfer turkey to a plate. Add parsley and mustard to vegetables in skillet; stir. Serve turkey and vegetables with a lemon wedge, if desired.