Note on mascarpone cheese: Grocery stores stock it in different sections; depending on the store, I’ve found it in the deli, the cheese section, or near the sour cream. If you can’t find it, substitute 1 1/2 ounces cream cheese and 1 tablespoon heavy cream.

Make ahead: Salmon fillets can be stuffed earlier in the day, covered and refrigerated.

Tips:
1. Drain and dry the spinach thoroughly. Excess moisture will make the stuffing runny. After draining, take handfuls of spinach and squeeze out as much moisture as possible. I don’t recommend using paper towels, as the chopped spinach tends to stick to them; a clean kitchen towel is another option.
2. Cut pockets carefully. Use a sharp knife and make a shallow, even cut from the top of each fillet. Avoid slicing through the bottom or ends to keep the filling secure during baking.
3. Check for doneness early to avoid overcooking. Ovens vary, so start checking the internal temperature at 10 minutes. Aim for a temperature of 135 to 140 degrees f. If you don’t have an instant-read thermometer, test with a fork—the salmon should gently flake.
4. Let it rest. After baking, let the salmon rest for 2 to 3 minutes to allow the juices to redistribute, keeping it moist and tender.