Wash guinea fowl and dab dry with kitchen paper. Season inside and outside with salt and pepper. Spread thyme inside the chicken and place on a fat pan or flat roasting pan.
Preheat the oven (electric cooker: 200 °C/circulating air: 175 °C/gas: level 3). Peel the onion, cut it into large pieces and distribute around the chicken. Roast in the hot oven for about 1 hour. After 30 minutes, pour 1⁄4 l lightly salted water over the guinea fowl.
Pour the liquid twice during the remaining cooking time.
Peel the carrots, leaving some tender green, wash. Clean and wash the mangetouts. Peel shallots and garlic. Finely dice garlic.
Caramelise the sugar in a large pan over medium heat until golden brown. Add shallots, immediately deglaze with 5-6 tbsp. water, vinegar and Marsala and bring to the boil while stirring. Add garlic and cook everything for about 12 minutes.
After about 8 minutes add the carrots. Season everything with salt and pepper.
Keep the guinea fowl warm. Pour the roast stock through a sieve directly into the vegetables. Bring to the boil again, add sugar snap peas and simmer for about 5 minutes. Season to taste with salt and pepper. Arrange guinea fowl and vegetables with stock.
Roast potatoes go well with it.