How to Make It
Melt duck fat in a cast-iron skillet over medium-high. Add onions and garlic, and cook, stirring often, until lightly caramelized, 8 to10 minutes. Reduce heat to medium-low; add paprika, salt, and cloves, and cook, stirring often, until rust color and very tender, 8 to 10 minutes. Remove mixture from skillet, and cool completely, about 20 minutes.
Combine onion mixture, rice, and pork belly in a bowl. Pour pork blood over pork belly mixture, and stir until well combined.
Cut casings into 6 (2-foot-long) sections. Run water through each section, making sure there are no tangles or holes. Tie a knot in 1 end of each casing section. Attach opposite end to the spout (at least 1/4-inch in diameter) of a funnel. Divide pork belly mixture into 6 equal portions. Spoon 1 portion pork mixture into funnel; use the handle of a wooden spoon to plunge pork belly mixture into casing, ensuring it goes through spout into casing. Repeat process with remaining casing sections and pork belly mixture.
Bring a large pot of water to a simmer with a temperature of 160°F. (It is important to maintain a water temperature of 160°F. If it goes above 170°F, the blood will curdle when cooked.) Carefully add sausages to water, in batches if necessary, and cook without disturbing 15 minutes. Remove sausages; roll into a coil on a large platter or baking sheet, and refrigerate, uncovered, 8 hours or overnight to set.
Slice sausages crosswise into 1/2-inch-thick rounds. Heat oil in a cast-iron or nonstick skillet over medium-high; add sausage rounds, and cook until browned and warmed through, about 3 minutes per side.
Note: You can also grill the links whole over medium-high heat and slice to serve.