Cook the spaghetti as per packet instructions; drain and keep aside. Reserve 1 cup of the drained pasta water.
To make the white sauce, melt butter in a large pot and add the flour. Mix well and cook on low heat to prepare the roux (this is essentially cooking for a minute or two to remove the raw taste of the flour). Add 1 cup milk and mix into the roux using a whisk. Continue cooking stirring constantly so that no lumps are formed. As the sauce begins to thicken, add another cup of milk and whisk well. Add the seasoning and cheese; continue to stir and when the sauce thickens again, add the third cup of milk. Whisk well and cook till the sauce attains a creamy consistency. Keep aside.
In another pot, heat olive oil and add the onions and bell pepper; sauté for a minute till the onions are just softened and add the sliced sausages. Cook for 1-2 minutes till the sausages are lightly fried and then add the seasonings.
Mix well and add the white sauce followed by the cooked spaghetti. Mix well to incorporate and if the sauce feels too thick, add some of the pasta water to loosen it up.
Serve immediately with grated parmesan on top.