Preheat oven to 200°C. Boil pasta until al dente and set aside. In a pan, heat up about a tablespoon of olive oil on medium heat. Add in the onion and cook, stirring frequently, about 10 minutes until soft and caramelised. Add in the garlic and balsamic vinegar. Add in the spinach and stir until wilted. Add in salt and pepper to taste.

Remove from heat and set aside. In a saucepan heat up the olive oil and add in the flour. Whisk together. While whisking continuously, slowly add in the milk. This will avoid lumps of flour. You may not need all the milk so only add half to begin with. Add in the mozzarella and stir.

If you want a thinner sauce, add in the rest of the milk. Season with salt and pepper to taste. Stir in the pasta and the onions and spinach. Pour into an 8x8 inch baking dish and top with more mozzarella.

Place in the oven for 25-30 minutes, until the cheese is slightly browned on top. Cool for 10 minutes and then serve. Can last up to 4 days covered in the fridge.