Put the pasta into a large pan with salted boiling water cook for 10 to 15 minutes or until al dente.
While pasta is cooking put the olive oil in a pan and fry the onion and green chillies until soft; stir in the smoked sausage and mushrooms and cook for 5 minutes. Stir in sweetcorn and set aside on a low heat with lid on pan.
To make cheese sauce, melt the butter in a saucepan and add flour to form a roux; cook for 2 minutes to cook out the flour. Add milk slowly, whisking continuously until sauce is thickened. Simmer for 5 minutes, remove from heat, add cheese sauce, mustard and salt to taste and put back on low heat.
Drain pasta; return to pan. Stir in the sausage mixture and cheese sauce well. Serve in bowls with grated cheese on top.