Wash the fillet and dab dry. Heat the oil in a pan. Brown the fillet in it all around for about 5 minutes. Season with salt and pepper. Fry for about 15 minutes at medium heat until done.
Turn it over once in a while. In the meantime clean, wash and cut the leek into long narrow strips. Blanch in boiling salted water for 2 minutes. Put the pasta in boiling salted water, pull and drain for 2 minutes.
Drain the peppers and cut them into strips as well. Wrap fillet in aluminium foil. Melt the butter in the frying fat. Swivel the noodles, leek and bell peppers in it. Season with salt and pepper. Cut fillet into slices and arrange on the noodles.