Heat a deep frying pan or casserole and fry the bacon until crisp and golden. Add a little oil if needed. Set the bacon aside.
Season the chicken with salt and freshly ground pepper on both sides. Brown it until golden in the same pan over a medium heat - skin side first. Set the chicken aside.Pour off some of the fat from the pan if there is a lot.
Fry the mushrooms over a good high heat until well browned and their liquid has evaporated, then add the onion and garlic and fry until the onion is translucent.
Stir in the tomato purée and cook it through for a couple of minutes, then sprinkle over the flour and stir well to combine.
Pour in the white wine and let it reduce for 5 minutes - stir so it doesn't catch on the bottom. Then add the stock along with the bay leaf, thyme sprigs and the crisp bacon, and stir to combine.
Return the chicken to the sauce and leave everything to simmer gently, covered, for about 45 minutes - depending on the size of the legs. The chicken doesn't need to be completely covered by the sauce.Check the temperature of the legs to make sure they are cooked through - if they are very large, they may need a little longer.You can also turn the legs once during cooking; the skin isn't meant to be crisp.
Before serving, stir the fresh tarragon into the sauce and season to taste with salt and freshly ground pepper.
Serve with mashed potatoes, boiled potatoes, rice or pasta, for example.