Poussin is the French word for a very young, small chicken. Here, they are enlivened by fresh thyme and oregano.
In a small bowl, whisk oil, vinegar, and mustard until well combined. Whisk in garlic, chives, thyme, oregano, dry mustard, salt, and pepper; set aside. Rinse poussins well under cold water. Pat dry.
Using kitchen shears, cut along either side of backbone; remove it. Turn over; gently flatten. Place marinade in a large resealable plastic bag; add poussins. Seal; turn bag to coat them with marinade. Refrigerate at least 2 hours or overnight.
Preheat oven to 400 degrees. Position a rack in center. Place a flat roasting rack in a shallow roasting pan or on a rimmed baking sheet. Coat rack with spray; set aside.
Remove poussins from marinade, letting excess drip off; place on rack, breast side up. Tuck wing tips under body. Sprinkle with salt and pepper; roast until golden and cooked through, about 50 minutes. Cut each in half through breast; serve.
Cornish hens can also be used.