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Place the top rack so it's about 3 to 4 in (7.5 to 10 cm) below the heating element. You'll also need to get out a baking sheet. Get out 1 pound (0.45 kg) of blood sausage links and set them on a cutting board. Use a sharp knife to slice each link in half lengthwise and then place them on the sheet so the cut-side faces up. Use a rimmed baking sheet to prevent the sausages from sliding off as you move the sheet. Put the sheet on the rack under the broiler and heat the sausages until they're browned on top. Watch the sausages as they broil to prevent them from burning. Since blood sausages are already cooked, you're simply heating them up. Pull the baking sheet from under the broiler and use tongs to flip each sausage over. Get out 2 tablespoons (30 g) of coarse-grain mustard and spread a little of it over each sausage link. Although you can substitute other types of mustard, the coarse-grain mustard will give the best texture to the sausages. Return the sheet to the top rack under the broiler and heat the sausages until they begin to brown and the mustard heats up. Then turn off the broiler and remove the sheet. Put several handfuls of fresh, stemmed watercress into a large bowl. Pour 1 tablespoon (15 ml) of sesame oil into the bowl and use your hands or salad tongs to toss the watercress. Add salt and pepper according to taste. You can add the remaining 1 tablespoon (15 ml) of oil if the watercress isn't coated. Divide the watercress salad between 4 serving plates. Place 2 of the blood sausage links on the greens and consider serving the meal with cooked apples or apple compote. Store leftover blood sausage in the refrigerator for up to 3 to 4 days. Avoid storing the watercress salad because it will soften too much as it's stored.
Preheat the broiler to HIGH and move a rack to the top. Cut the sausages in half lengthwise and place them on the sheet. Broil the sausages for 1 to 2 minutes. Flip the sausages and cover them with coarse-grain mustard. Broil the sausages for another 1 to 2 minutes. Mix a quick watercress salad with sesame oil, salt, and pepper. Serve the broiled blood sausage with the greens.