Poach the chicken in water – place the chicken breasts in a small saucepan on a high heat and pour in 500ml of cold water. When the water begins to boil, lower the heat and gently simmer for 14 minutes (turning once, halfway through cooking). Remove the chicken and set it aside to cool on a plate, before shredding. Discard the poaching liquid.
Put your oven on at its lowest temperature.
While the chicken is poaching, heat the olive oil over a medium heat in a medium saucepan. Add the chopped onion and cook for 3 minutes, stirring frequently, until translucent.
Add the chicken stock pot or crumble stock cube, sugar and chipotle paste. Stir to combine, then add the tin of tomatoes and bay leaf. Stir and bring to a simmer. Cover with a lid and simmer for 15 minutes. With 5 minutes to go, add the shredded chicken. Remove from the heat and discard the bay leaf.
Heat a large frying pan over a medium low heat and brush it with a drizzle of oil. Lay a tortilla in the pan and arrange a quarter of the chicken tinga mixture over half the tortilla and top with a quarter of your choice of cheese(s).
Fold the other half of the tortilla over the filling to make a semi-circle and cook for 2 minutes until the cheese is starting to melt and the tortilla is lightly browned. Carefully turn the quesadilla over using a fish slice or palette knife, and fry on the other side for 2 minutes until it's golden.
Transfer it to the oven to keep warm while you make the rest of the quesadillas.
Serve, cut into wedges with your choices of Mexican sides.