Preheat oven to 375°F.
Combine the softened butter with garlic salt, smoked paprika, sage, thyme, onion powder, sea salt, black pepper, and oregano.
Pat the chickens dry, season the cavities with additional salt and pepper, and add the lemon slices.
Carefully loosen the skin over the breasts and thighs, then rub the compound butter generously under and over the skin and around the entire chickens.
Place the chickens in a large roasting pan, sheet tray, or cast-iron skillet and roast for 20 minutes.
Remove the chickens temporarily, add the potatoes, carrots, and onions to the pan, and toss with the rendered butter and chicken drippings.
Season lightly with salt and pepper, then place the chickens back on top.
Roast for approximately 1 hour and 15 minutes more, or until the thickest parts of the breasts and thighs reach at least 165°F.
Rest the chickens for 10–15 minutes and transfer the vegetables to a serving platter.
Place the roasting pan over medium heat, whisk 2 tablespoons flour into the drippings and cook for 1–2 minutes.
Gradually whisk in 2 cups chicken stock, scraping up the browned bits, and simmer until smooth and thickened.
Add freshly cracked pepper and adjust salt as needed.
Carve the chickens and serve with the roasted vegetables and pan gravy.
