To prepare the beef mixture, heat the oil in a frying pan and sauté the onion until lightly golden. Stir in the smoked paprika, cumin and coriander and cook for another minute or so.
To make the chilli salsa, place the tomatoes in a non-metallic bowl with the red onion, chopped coriander, chilli, lime juice and olive oil, stirring until well combined. Season to taste, cover with clingfilm and leave at room temperature to allow the flavours to develop.
Preheat the oven to 200C (400F), Gas mark 6 and heat a griddle pan over a medium heat until very hot. Brush one side of each tortilla with a little of the olive oil. Place one tortilla in the pan, oiled-side down, and cook for 1 minute until nicely marked, pressing down with a spatula. Repeat with the remaining tortillas.
Arrange half the tortillas on baking sheets, marked-side down. Divide the minced beef mixture among them and then sprinkle the cheese and spring onions on top. Season to taste. Cover with the remaining tortillas, marked-side up, and bake for about 5 minutes or until heated through and the cheese has melted. Allow to cool slightly until easy to handle.
Cut each quesadilla into eight wedges with a serrated knife, pizza cutter or kitchen scissors. Garnish each wedge with a small spoonful of soured cream and a coriander leaf. Arrange on warmed plates or one large platter to serve, with a separate bowl of the chilli salsa to hand around.