Preheat the oven to 450°F. Butterfly salmon fillet through the center, without cutting all the way through, so that it lays open flat in one large piece like a book. Turn salmon over so skin side is facing up and place on a clean work surface. In a bowl, mash together feta and ricotta until well blended.

Season salmon with 1/4 teaspoon of the salt and 1/4 teaspoon of the pepper, then spread fillet with mustard. Arrange half of the spinach on top, leaving a 1-inch border around the edges. Evenly dot cheese mixture on top of spinach, then top with remaining spinach. Starting from one of the long sides, gently roll up salmon and tie snugly with cooking twine at 1-inch intervals.

Gently transfer salmon to an oiled baking sheet. Brush top and sides of salmon with oil and season with remaining 1/4 teaspoon salt and 1/4 teaspoon pepper. Scatter dill and oregano over top and sides. Roast until salmon is just cooked through, about 20 minutes. Let rest for 5 minutes, then carefully transfer to a serving platter, remove and discard the twine and serve.