Rinse chicken and pat dry, then arrange in a single layer in a 9- by 13-in. baking dish. Scatter 1 1/2 tbsp. garlic slices over chicken, then sprinkle with 1 1/2 tbsp. rosemary. Drizzle chicken with olive oil and sprinkle with 1 tsp. salt and 1/2 tsp. pepper. Turn to coat; chill, covered, at least 6 hours and up to overnight.
Prepare a gas or charcoal grill for indirect medium heat (about 400°; you can hold your hand 5 in. above cooking grate only 5 to 7 seconds). If using gas, turn all burners to high and close lid; heat 10 minutes and turn off one of the burners. The area over the turned-off burner is the indirect heat area. If using charcoal, light 60 briquets on firegrate. When coals are covered with ash, about 20 minutes, bank on opposite sides of firegrate and let burn to medium. The area over the cleared section is the indirect heat area.
Bring a large pot of salted water to a boil. Add shallots and cook until tender, 10 to 15 minutes. Transfer to a bowl and let cool; cut any large pieces into halves or quarters. Bring water back to a boil, add peas, and cook until just tender, 2 to 4 minutes. Drain peas; transfer to a bowl of ice and cold water. Drain again.
Pat chicken thighs dry and lay them skin side down over indirect heat area; close lid. Cook 10 minutes (12 for large thighs). Turn over and grill, covered, 10 minutes more. Move chicken over direct heat and cook, turning once, until skin is well browned, 3 to 5 minutes, and until chicken is no longer pink inside (cut to test). Transfer the chicken to a pan; tent pan with foil to keep warm.
In a large frying pan, cook pancetta over medium heat until fat begins to render and pancetta starts to become crisp, about 5 minutes. Add remaining garlic, rosemary, salt, and pepper, the onion, parsley, and shallots to pancetta. Cook, stirring, 2 minutes. Stir in peas and butter and cook 2 minutes, or until peas are heated through.
Divide chicken among 6 warm plates. Sprinkle lemon juice over vegetables. Stir and spoon onto plates alongside chicken.
*If using frozen peas, don't cook them in step 3; just add to pancetta mixture in step
Note: Nutritional analysis is per serving.