Zest the 2 lemons, then juice only one of them (leaving the remaining whole). Pour that into a large bowl. Add oil, Dijon, and 2 cloves of minced garlic and whisk vigorously until combined. Season with some salt and pepper.
Toss each the chicken and potatoes with dressing to completely coat, then spread out onto a large rimmed baking sheet in a single layer, placing chicken skin side up. Sprinkle with rosemary.
For remaining lemon, peel lemon with a sharp knife removing as much of the white pith as possible. Thinly slice the lemon and add to the pan. Strip the excess paper layers of the remaining head of garlic and slice of the top of the head to expose a bit of each clove. Drizzle with oil and place in pan.
Roast at 425°F for 40-50 minutes, until chicken is fully cooked, turning potatoes once during cooking to ensure even browning.
To serve, squeeze out baked garlic cloves.
Using a baking pan with will speed browning due to quicker evaporation. Using a deep baking dish allows to much moisture to remain, thus taking much longer to achieve the proper browning.