Step 1 Use poultry shears or a sharp knife to remove the backbone and wing tips from each poussin at least two hours before cooking. Flatten the birds until they lie completely flat. Using the tips of your fingers, carefully lift the skin away from the flesh on each breast and the legs to make a small pocket. Divide the chopped ramps among the pockets. Season both sides of each bird with one-half teaspoon salt and one-fourth teaspoon pepper, or to taste. Lay on baking sheets and place in the refrigerator for 1 1/2 hours to dry the skin slightly.
Step 2 Remove poussins from refrigerator 30 minutes before cooking. Heat the broiler or grill.
Step 3Lay the birds skin side up on an oiled broiler pan as far as possible -- approximately 6 inches from the heat source. If the broiler will not fit all the poussins at once, work in batches; they can be served hot or at room temperature. Cook 12 to 15 minutes until dark brown, then flip over and continue cooking until the legs can be easily moved, about 10 minutes longer. Watch carefully during cooking to make sure the birds do not burn. Remove from the broiler and let stand 5 minutes.
Step 4While the birds cook, combine the lime juice and mustard in a salad bowl and whisk to blend. Whisk in the olive oil and season with salt and pepper to taste.
Step 5Toss the watercress in the bowl with the dressing and divide among 8 plates. Top each with a poussin and serve.