On a medium heat cook the onion until nicely softened and golden, then set aside.
Slice the beef into strips and then lightly fry in the same pan as you cooked the onions. Season the meat with salt and pepper.
Add the mushrooms when the meat is starting to brown, stir until the mushrooms start to brown a little then add the onions back to the pan and stir.
Add the cherry tomatoes and cook until the tomatoes soften. Add the peppers and stir until they soften. At this point, stir in horseradish.
If you wish you can remove this from the heat now and set aside to let the flavours mingle. This is helpful if you are precooking for a meal later. If you want to eat straight away, then add enough sour cream to make a nice sauce with it. Stir through on a medium heat and season if required.
If you want some extra flavour add a small amount of fresh chilli at stage 2.