Drizzle a bit of olive oil in a large sauce pan. Add the bacon and set the pan over a medium heat; cook stirring occasionally, until brown and crisp, 8 to 10 minutes. Remove from pan and drain on paper towels. Do not discard bacon grease.

Add butter and onion to pan with bacon fat. Season with salt and cook until the onion are soft and aromatic, 4 to 6 minutes. Add the flour and cook, stirring frequently, until the mixture looks like wet sand about 3 minutes.

Slowly whisk in milk and mustard. Season with salt and bring to a boil, then reduce the heat to low. Simmer until the mixture is slightly thicker than heavy cream, 6 to 8 more minutes. Bring a large pot of water to a boil. Add the pasta and cook 1 minute less than the instructions on the package suggest. Drain pasta.

Add the cheeses to the milk mixture and whisk to combine. Taste and adjust the seasoning, add more milk if mixture is too trick. Stir in bacon and pasta. Serve immediately.