1.Preheat oven to 180°C/160°C fan. Oil a deep 2-litre (8-cup) ovenproof dish.
2.Cook pasta in a large saucepan of boiling water for 1½ minutes less than cooking time on the packet; drain. Cook bacon in same pan, stirring, until crisp; drain on paper towel.
3.Melt butter in same pan, add flour; cook, stirring, for 1 minute. Gradually stir in milk; cook, stirring, until sauce boils and thickens. Stir in cheeses and mustard.
4.Combine pasta, cheese sauce, bacon and spinach in a large bowl; pour into ovenproof dish. Dot with a little extra butter. Bake for 30 minutes or until browned. Top with dill and zested lemon.
When matching sauces with pasta, a simple rule is the longer the pasta, the thinner the sauce; the shorter the pasta, the thicker or chunkier the sauce.Note