If you run out of time to assemble the sandwiches, simply set out the spiral-cut ham along with the relish, mustard and rye bread, and let people assemble their own.
Heat oil in a 12-inch skillet over medium-high heat. Add onions; saute until well-browned, 10 to 12 minutes. Add rosemary and cloves; continue to saute until fragrant, 1 to 2 minutes longer. Stir in cranberry sauce, and simmer until heated through. Remove from heat and set aside. (Cranberry-Onion Relish can be cooled, covered and refrigerated up to 2 weeks ahead.)
To assemble: Working in batches, lay breads on a work surface and spread with about 1/4 tsp. mustard and 1 tsp. cranberry relish. Top half the breads with a portion of ham, cheese and arugula (optional), then the remaining bread slice. Halve each sandwich on the diagonal, sticking each half with a toothpick. (Sandwiches can be covered with a damp paper towel and plastic wrap and can remain at room temperature for 2 hours.)
Per Serving: 114 calories; protein 6.5g; carbohydrates 10.9g; fat 4.9g; cholesterol 18.9mg; sodium 388.7mg.