Position the racks in the top and bottom third of the oven; preheat to 400°. Grease a rimmed baking sheet liberally with lard. In a large bowl, whisk the flour, baking powder and salt. Make a deep well in the center. Place 1/2 cup lard and the buttermilk in the well. Using your fingertips, mix the lard and buttermilk together. Gradually begin mixing in the flour a little at a time, until a wet dough forms. Move it around in the bowl, turning occasionally and introducing more flour with each movement until the dough is soft and tender but not sticky. There may be about 1/2 cup flour left in the bowl.
Pinch off about 3 tbsp. dough; round it out, pinching the bottom together. Place in the corner of the greased baking sheet and press to 1/2 inch thick. Repeat with the remaining dough, arranging with the sides of the biscuits touching. It’s OK if you don’t fill the entire tray. It’s more important that the biscuits be touching each other.
Place in the lower third of the oven and bake until beginning to brown, about 18 minutes. Rotate the tray and move to the top rack; bake until golden brown and crispy on the bottom and light golden on top, about 7 minutes more. Let cool slightly.
While the biscuits bake, shape the sausage into 10 thin patties that are a little wider than the biscuits. Heat a large skillet over medium and fry the patties until browned and cooked through, 3 to 4 minutes per side. Split the biscuits in half. Spread a little mustard on the bottom and grape jelly on the top. Tuck a sausage patty into each biscuit.