Begin by preparing the sauce. Grate tomatoes on the large side of a box grater into a bowl. Add in garlic and chilis. Season to taste with salt, pepper, and olive oil. Set aside.
For the filling mix the ricotta, pecorino, basil, salt and pepper until combined. Transfer to a piping bag or a ziplock bag. Fill each of the cleaned squash blossoms with the filling. Fill them about 3/4 of the way, and gently pinch the top of the blossms to close them a bit. If you fill them all the way up, the filling could seep out during frying!
Heat about 2 inches of canola oil in a large pot for frying. You are looking for the oil to reach around 350 F.
While oil heats, in a medium bowl, whisk flour, seltzer, and salt until just combined. A few lumps are ok!
Once oil is hot, dip filled squash blossoms, one at a time, to coat. Gently drop in oil and fry until lightly golden, about 3-5 minutes, turning over as needed to fry evenly. Fry in batches so you don't overcrowd the pan.
Sprinkle with a touch of salt immediately after frying.