Heat oven to 200C/180C fan/gas 6. Toss the sweet potatoes with 2 tsp of the oil, half the oregano and some seasoning, and arrange in a single layer on a flat baking tray lined with baking parchment.
Put 1 tsp of the oil into a food bag with seasoning, add the onion rings and shake to coat. Add the polenta and shake again. Arrange on another tray and cook with the fries for 35 mins, turning halfway.
Mix the remaining oil, the parsley, lemon zest and juice, garlic, vinegar and oregano with seasoning, and brush half over the steaks. Pan-fry for 30 secs each side, serve with fries, onion rings and remaining dressing.