This recipe seems to be out of context as it refers to milk that was “put in with the rabbit” but there is not a rabbit recipe yet. If this was for a class, perhaps the students learned all the recipes and then created their own cookbook as their final project? It makes some sense to me. The bread sauce appears to be like a gravy.
Put into a clean saucepan 1 pint milk, taking the milk that was put in with the rabbit for 20 min. Throw into this an onion & let it stand by the side of the fire for about 10 minutes. Take out the onion & shake in as many bread crumbs as will thicken it. Add a pinch salt & a grate or two of nutmeg.