Preheat oven: Heat oven to 450°F.
Grind the spices: Add fennel seeds and whole black peppercorns to a mortar and pestle. Grind until finely crushed into a powder. Mix with oregano, thyme, and salt.
Season chicken: Pat chickens dry. Rub generously with olive oil and season with the freshly ground herb and spice mixture.
Prepare pan: Add sliced lemon and onion to a large roasting pan. Place chickens on top and scatter garlic around them.
Add stock: Pour about 1/2 cup chicken stock into the bottom of the roasting pan, around the chickens.
Roast: Roast for 45–55 minutes, or until the thickest part of the breast reaches 165°F.
Rest: Remove from oven and rest for 10–15 minutes before carving.
Cook potatoes: Boil potatoes in salted water for 10–15 minutes, until fork-tender. Drain.
Make dressing: Mix parsley, red onion, olive oil, lemon juice, oregano, fennel, pepper, and salt in a large bowl.
Toss: Add warm potatoes and gently toss with the dressing.
Serve: Carve chicken and serve with the herby potatoes. Spoon roasted garlic, onions, lemon, and pan juices over the chicken.