Preheat the oven to 400 degrees F. Line 2 baking sheets with parchment. Set aside.
In a small bowl, stir together the mascarpone cheese, dill and zest and juice of the lemon to combine. Add salt and pepper to taste. Set aside one-third of the filling and reserve for the sauce
Pat a salmon filet with a paper towel to slightly dry. Place the filet at one end of a strip of phyllo, leaving a 3- to 4-inch overhang, and season with salt and pepper. Spread half of the remaining mascapone mixture over the filet then cover with an even, flat layer of the half the spinach leaves. Wrap the phyllo overhang over the filet and roll the salmon up in the dough, brushing the phyllo with butter as you roll. Tuck any excess phyllo under. Prepare the second piece of salmon the same way and place on one of the prepared baking sheets.
Fold the last piece of phyllo 4 times to a quarter of its original size (an approximate 12-by-9-inch rectangle). Brush with butter and, using a 2-inch heart cutter, cut 6 hearts from the phyllo. Place on the other prepared baking sheet.
Bake until the hearts are golden, 5 to 10 minutes, and the phyllo around the salmon is crisp, flaky and golden, about 20 minutes.
In a small saucepan, whisk together the reserved mascarpone mixture, the cream, Dijon mustard and salt and pepper. Warm (do not boil) over medium heat. Serve the salmon packets with a drizzle of cream sauce and top each with 3 hearts. Serve extra sauce on the side.
Yield:1 large (36-by-36-inch) sheet phyllo dough
In a large bowl, whisk together the flour and salt. In a liquid measuring cup, whisk together the water, oil and egg. Pour the wet ingredients into the dry and using a wooden spoon or a Danish whisk, stir the mixture until a shaggy dough comes together.
Transfer the dough to a work surface and knead until the dough is a bit smoother. Then "slap" the dough and turn it over and knead it until smooth; this could take between 5 to 10 minutes. You can also mix the dough in a stand mixer with the dough hook attachment. This takes about 10 minutes. Cover the dough in plastic wrap and allow to rest a room temperature for 2 hours.
Cover a table (at least 36 inches square) with a clean tablecloth. Sprinkle the cloth all over with flour. Gently press the dough into a rough 12-by-12-inch square and then gently pull the dough into a large square, about 36 inches. Use a pair of clean kitchen scissors to cut the thicker edges off the perimeter and discard them. Allow the dough to rest to dry out just a bit, about 5 minutes.