In a firepit, build a wood fire about as wide as the cooking grate. Let it burn to low flames and embers (1 to 1 1/2 hours; or use charcoal*); fire is ready when you can hold your hand 5 in. above grate only 5 to 7 seconds. (Watch us build a fire: sunset.com/cookingfire.) Cut 6 sheets of heavy-duty foil into 18-in. squares. Brush or rub center of each with a little oil; set on work surface.
In a large bowl, combine bread crumbs, milk, grated parmesan, chopped parsley and basil, 1/2 tsp. each salt and pepper, and the chopped Calabrian chile. Add ground turkey and sausage (squeeze it from casings if it's in links). Stir until well blended, then shape into 18 balls, setting on parchment paper or plates.
Put 3 meatballs in the center of each foil sheet, then divide potatoes, onion, and carrots among sheets. In a small bowl, combine remaining 1/4 cup oil, 1/3 cup water, the cornstarch, and remaining 1/2 tsp. each salt and pepper; spoon over ingredients on foil, stirring sauce as you go.
On each bundle, bring 2 opposite sides of foil together across the middle and fold to make a seam 1 in. wide. Fold seam on itself again, then fold it flat on packet. Fold ends on themselves about 1 in., then repeat to seal securely.
Set bundles on grill and cook, turning over with tongs every 10 minutes, until vegetables are tender and meatballs are cooked through, about 30 minutes (to check, open with a knife).
Protecting hands with grilling gloves, open packets (or snip off ends of foil with scissors) and scoop food onto plates. Snip whole parsley and basil leaves on top and serve with more chiles and shredded parmesan.
*For a charcoal fire, ignite a full chimney of briquets once you've made the meatballs, and let burn until covered with ash. Spread beneath cooking grate, then ignite a second chimney to add during cooking if needed to maintain a medium fire (see step 1); cooking may take up to 1 hour.
Make ahead: Freeze meatballs (step 2) on baking sheets lined with parchment paper until solid, then transfer to resealable plastic bags, double-lined with parchment. Pack frozen in your cooler and thaw before cooking (takes 1 to 2 days). You can also chill packets (through step 4) in cooler up to 6 hours.