In a small bowl, combien the garlic, olive oil, lemon, parsley, salt and pepper. Remove any giblets from inside the chicken, pat dry and rub with the marinade including in the cavity. Cover and marinate overnight.
Preheat the oven to 400°F.
Arrange the chicken on a roasting pan. Roast for 90 minutes, or 20 minutes per pound, until cooked through, golden and juicy. Let the chicken rest for 10 minutes, covered, before serving.
To make the sauce, in a medium saucepan, melt the butter over medium heat. Add the onions and cook until soft, about 10 minutes. Add the rosemary, salt and pepper and cook for 5 more minutes. Add the lemon juice and cook for 2 minutes. Add the chicken stock and bring to a boil. Reduce the heat to a simmer and cook for 10 minutes.
In a small bowl, combine the cornstarch with 1 tablespoon water into a smooth paste. Whisk the cornstarch slurry into the sauce and bring to a simmer for another 10 minutes, until thickened.
On a baking sheet, toss the swiss chard, mushrooms, and onion with the olive oil and a big pinch of salt and pepper. Roast until the mushrooms are soft, about 10 minutes.
Meanwhile, bring a large pot of salted water to a boil. Add the orzo and cook according to package instructions. Drain the orzo, return to the pot, and drizzle with olive oil.
Add the roasted vegetables and roasted red peppers to the orzo and toss to combine. Season to taste.
Serve the chicken with orzo salad and lemon rosemary sauce.