Cook pasta, according to package instructions, one to two minutes fewer than directed. Drain and set aside.Cook pasta, according to package instructions, one to two minutes fewer than directed. Drain and set aside.
Meanwhile, cut bacon pieces in half vertically, then into small bite sized pieces. Fry until crispy. Drain, and set aside.
In large French or Dutch oven, melt butter. Whisk in flour and cook over medium heat until golden. Slowly whisk in milk while cooking over medium heat.
Stir in mascarpone cheese, whisking until melted and creamy. Whisk in fontina and parmesan until melted and creamy. Add nutmeg, cayenne, black pepper and salt (if desired). Bring to slow simmer, and stir in pasta and spinach. Cook just until heated through.
Serve immediately, sprinkling each portion with bacon.