Heat a pan over medium-high heat. Add oil and butter. Allow it to melt.
Once it starts bubbling, add chopped onion and cook for 2-3 minutes. The onions should become soft and translucent. Then add the chopped garlic and cook until the raw smell disappears. Keep stirring occasionally.
Add the sliced chicken sausages and cook for a few minutes. The sausages should become light golden in colour.
Then add all the chopped bell peppers, Italian seasoning, salt and black pepper powder. Give it a toss and cook for a minute or two. Turn the heat off and keep it aside. Preheat oven to 180-degree Celcius.
Heat water in a pot over a high flame. Once it starts boiling, add salt and pasta. Give it a stir and cook over a medium flame until al dente. Then drain the pasta and add it directly to the cooked vegetables.
To make the white sauce, melt butter in a sauce over medium flame.
Add all-purpose flour and mix well.
Continue to cook for a few minutes until it is fragrant. It may take 2-3 minutes.
Pour a little milk, give it a good whisk. Make sure there are no lumps. Then pour the rest of the milk. Whisk again. Bring it to a boil and once it starts thickening, add garlic, onion and black pepper powder. Season it with a little salt. Mix well and cook for a few seconds.
Turn the heat off and directly pour the sauce over the pasta. Give it a gentle mix.
Transfer the pasta to a baking dish and sprinkle grated mozzarella cheese over the top.
Keep it in the oven and bake at 180-degree Celcius for about 10-15 minutes or until the cheese melts and becomes slightly golden in colour.
Serve it immediately. If you like then you can also store it in the refrigerator for up to 3 days.