This recipe requires an extra bit of effort but, the gnocchis and sauce can be prepared a couple of days in advance. The final preparation takes about 5 minutes and only fifteen more minutes are required for cooking.
Butter an 8 inch square mold and line the bottom with parchment paper. Set aside.
Pour milk into a non stick casserole, if possible. Heat on medium-high.
Place the semolina in a bowl with the ground mace and blend well.
Pay attention to the milk and when it reaches the boiling point, incorporate the semolina – sprinkling lightly – while constantly mixing with a whisk.
Mix until a thick coat forms. Continue cooking on low heat for 5 minutes, mixing constantly with a silicone spatula.
Remove from the heat and incorporate half the butter, Gruyère and half the Parmesan with a spatula. Incorporate the yolks one at a time.
Spread the semolina/milk coat evenly in the buttered mold. Place a plastic sheet on top to assist in levelling the mixture.
Refrigerate for at least one hour (can be done two days beforehand).
Heat the oven to 450 °F. Cut the gnocchis into 2 – inch x 4 – inch rectangles.
Pour the tomato sauce into a gratin dish. Place the gnocchis on the sauce so that they lightly overlap.
Melt the remaining butter along with the sage in a small pan. Brush the gnocchis with the melted butter. Sprinkle on the remaining Parmesan and bake for 15 minutes in the oven until hot and golden.