Scrub the mussels under cold water and pull off the beards. Discard any with cracked shells or that don't close when tapped firmly on the counter.

Using a small paring knife, pry each mussel open at the hinge without fully separating the two shells, so they stay attached like a book. Set the opened mussels on a tray while you make the filling.

Tear the stale bread into a bowl and pour the milk over it. Let it soak for 5 minutes, then squeeze it firmly dry with your hands and crumble it back into the bowl.

Add the eggs, grated pecorino, chopped parsley, minced garlic, salt, and black pepper to the bread. Mix with a fork until you get a thick, cohesive paste that holds together when pressed.

Spoon about a tablespoon of filling into each opened mussel, pressing it gently between the two shell halves so the mussel closes around the stuffing.

Wrap kitchen string around each stuffed mussel twice and knot it snugly, or secure with a toothpick through both shells, so the filling stays enclosed during cooking.

Warm the olive oil in a 30 cm / 12-inch skillet over medium heat. Add the sliced garlic and chili flakes and cook for 1 minute until fragrant, not browned.

Pour in the crushed tomatoes and season with salt. Bring to a gentle simmer and cook uncovered for 8 minutes to concentrate the sauce slightly.

Nestle the stuffed mussels into the sauce in a single layer. Cover the skillet and simmer on low for 15 to 18 minutes, until the filling is firm and opaque and reaches 165 F / 74 C in the center.

Remove the string or toothpicks from each mussel with tongs. Scatter torn basil over the top and serve directly from the skillet with crusty bread.