Dust the chicken in the seasoned flour. Heat the oil in a large, heavy-based casserole over a medium-high heat and fry the chicken until golden brown. Remove and set aside.
Reduce the heat, add the onion and peppers and cook, stirring frequently, for 4–5 minutes until softened and golden. Stir in the garlic, paprikas and prosciutto and fry for a further 1–2 minutes.
Return the chicken to the pan, pour in the Marsala, wine, 100 ml (3½ fl oz) water and the tomatoes. Stir in the thyme and season to taste. Bring to the boil, then reduce the heat to a simmer, cover the pan and leave for 30–35 minutes until the chicken is cooked and the sauce is rich and thick.
Serve the chicken in bowls with lots of sauce.