Cook the pasta shells according to the directions on the packaging. Drain and keep some of the pasta water (you will need a later). Allow the pasta to cool. Find a dish that holds all the shells.
Melt the butter in a pan and cook the chopped onion until translucent, add the garlic and cook another minute. Add flour and then milk and/or pasta water or stock until you have something that looks like a thick sauce.
Add Provolone cheese, ham, peas, salt, pepper, and tarragon. You may want to add a little more liquid to make sure the cheese melts, but the sauce should remain thick. (NOTE 3) Allow the filling to cook.
Once the pasta and fillings are cooled, you will need to fill the shells. Make sure you fill them to the rim.
Pour a little bit of pasta water in the dish (not on the shells). This will prevent the shells from sticking.
Sprinkle the top with a decent amount of shredded Mozzarella cheese and place the dish in an oven of 400 degrees F for about 15-20 minutes. (NOTE 4)
Notes
4 ounces of jumbo pasta shells is about 18-20 shells (depending on size). This may depend on the brand you buy.
1 clove of garlic can be replace by 1 teaspoon of garlic paste.
A thin sauce will have the tendency to run out of the shells during baking, so make the sauce relatively thick.
If the top becomes too dark, but the dish is not yet heated enough, cover it with aluminum foil.