Clean, clean, wash and slice the mushrooms. Paprika as well. Quarter and cut finely. Peel, halve and slice the onion.
Melt the butter in a large frying pan and over high heat add the onions, peppers, garlic and mushrooms to the pan. Use two scrapers to stir the vegetables gently for about 10 minutes. Season a little later (draws liquid). Add the balsamic vinegar and let it steep for 5 minutes on a low flame. Put in a salad bowl and let cool a little.
Cook the spaghetti in salted water. Arrange the spaghetti on a plate, add the lukewarm salad and garnish with basil.