Tagliatelle with ham and mushrooms

• 1 medium-sized onion
• 4 oz (100 g) button mushrooms

Peel and chop the onion. Wipe and thinly slice the mushrooms and cut the ham in thin strips. Melt 1½ oz (40 g) of the butter in a frying pan over low heat, add the onion and cook gently for 10 minute