Heat oven to 200C/fan 180C/gas 6. Beat the mustard and butter together, then season well. Put the chickens into a large roasting tin; then, starting at the neck end, pull the skin away from the breast and thigh meat with your fingers. Stuff the butter under the skin, saving 1 tbsp for later. Put half the garlic bulb into each cavity. Season generously, cover with foil, then roast for 1 hr, uncovering for the final 20 mins, until crisp and golden.
Lift the birds onto a board, then sit the pan on the hob and splash in the wine. Reduce for 5 mins, scraping up the meaty bits with a wooden spoon, then add the stock and bring to a simmer. Tip in the carrots, cover with a sheet of foil, then cook for 5 mins. You can do this ahead of time. Add the peas and simmer for a few mins until tender. Just before serving, whisk in the reserved mustard butter, chopped herbs and any resting juices, then season to taste. If you like, transfer everything to a platter and bring to the table – or simply return the chickens to the roasting pan.