Combine sour cream, curry ketchup, and cream in a bowl. Whisk until smooth. Season to taste with curry powder.
Cut onions into thin rings. Wash pork fillet, pat dry, and cut into medallions 2 cm thick.
Heat oil in a large skillet over medium-high heat. Working in batches, sear pork medallions for 3-4 minutes per side until golden brown. Remove and season with salt and pepper.
In the same skillet over medium heat, cook bacon until crispy, about 8-10 minutes. Remove and break into pieces. Fry onion rings in the remaining fat for 2-3 minutes until softened, then season with salt and pepper.
Arrange pork medallions, bacon pieces, and onion rings in a baking dish. Pour the curry sauce over everything. Bake in a preheated oven at 200°C (392°F) with top and bottom heat for 30 minutes until the pork reaches an internal temperature of 63°C (145°F).
Serve with baguette, rice, or mashed potatoes and a fresh green salad.