Slice cabbage as thin as possible, soak in a bit of vinegar, and set aside.

Slice bell peppers thinly. Peel carrots into thin strips, then roughly chop. Slice leeks diagonally into long strips. Rinse and set aside.

Heat oil in a medium stir fry pan. Add red onion and stir briefly, keeping the color.

Add leeks, bell peppers, and carrots. Stir until just softened.

Add butter and mix.

Drain cabbage, add to pan, and fold in bit by bit to combine flavors and colors.

Once cabbage is just softened, quickly transfer to a serving dish to keep vegetables vibrant and not soggy.