Start by adding the rapeseed oil to a large, deep frying pan over high heat. Allow the pan to preheat, then add the spring onion, red pepper, and mushrooms.
Stir-fry on high heat for around 5 minutes, constantly stirring.
Meanwhile, combine the gochujang paste, soy sauce, honey, rice wine vinegar, and chicken or veggie stock in a bowl.
Reduce the heat to medium/low, and season the vegetables with salt. Add the gochujang mixture and garlic into the pan, stirring to combine.
Add the udon noodles to the pan and cover with a lid for 2 minutes. This will help loosen the noodles before stirring them into the sauce.
Remove the lid and stir the noodles into the sauce. Continue to cook for around 3 minutes, until the sauce has thickened/reduced and the noodles are cooked.
Top with the reserved thinly sliced spring onions and enjoy!