Francesinha sauce: add the butter and olive oil to a large pot set to around medium or just below medium heat. Once the butter is melted and bubbly, add the diced onion and cook for 5 to 7 minutes to soften the onion.

Once the onion is soft, add garlic and cook for 2 minutes.

After 2 minutes, add all other ingredients for the Francesinha sauce except for the salt, pepper, milk, and cornstarch. Bring the sauce to a simmer and cook for around 30 minutes.

After 30 minutes, remove bay leaves and if you have an immersion blender, you can blend the sauce to a consistent thickness. If you do not have an immersion blender, you can remove the pot at this point from the heat and blend the sauce in a traditional blender. But be careful because the heat in a blender can be a little explosive. You just want the sauce to be smooth.

Once the sauce is blended and smooth, add the pot back to medium heat and add cornstarch and milk. Bring it to a simmer and cook for another 20 minutes. At this point you should taste the sauce and see if it needs salt and pepper. Season it until it tastes good to you.

Warm up the sauce: if you aren't immediately making the sandwich, you can refrigerate this sauce for up to a week. Once it's time to make the sandwich add 2 large spoonfuls of the sauce (between 1/4 to 1/2 cup of sauce per sandwich) to a small pan and warm on the stove while you prepare the sandwich. 

Spicy butter sauce: Add butter and hot sauce to a small microwave-safe bowl and microwave for 20 to 30 seconds or until the butter is fully melted. Stir to make sure everything is incorporated well. 

Cachorrinho: prepare a panini press and set it to high. If you do not have a panini press you can cook this sandwich upside down on a grill pan or cast iron skillet set over medium heat. 

Slice your baguette, remove the casing from the uncooked sausage, and spread the meat on the bottom slice of the baguette.

Once the panini press is warmed and ready, add the bottom slice of baguette with the sausage spread on top and cook for 4 to 5 minutes. At this point, the sausage should be fully cooked and it should have some nice grill marks.

With a brush or spoon, paint some spicy butter sauce on the cooked sausage and then layer on 1 to 2 slices of cheese (don't use all the cheese—save at least half of it for a bit later) that has been broken in half to fit on top of the meat. Cover with the top of the baguette and close the panini press again.

Preheat your oven to 350 F or turn on your broiler. This is for the final cooking process which will melt the rest of the cheese. 

Cook the closed sandwich until the cheese starts to get nice and melty, probably another 3 minutes or so. Open the panini press and paint some more spicy butter sauce on top of the bread, close the press and cook one more minute. 

Add more cheese to the top: remove the sandwich from the panini press to an aluminum foil-lined baking sheet and cover the top of the sandwich with the rest of the cheese slices.  

Place the pan with the cheese-covered sandwich into the oven to cook until the cheese is very melty on top. If using the broiler, be careful to monitor it so that the sandwich doesn't burn.

Once the top of the sandwich is fully covered with melty cheese, remove it from the oven and place it on a plate or a bowl. Pour over some of the warm Francesinha sauce and serve.