For the salsa, combine the diced tomatoes with the red onion, garlic, chili pepper, cilantro, lime juice, salt, and pepper. Set aside.
For the guacamole, roughly mash the avocado. Mix in the onion, cilantro, lime, and olive oil. Season with salt and pepper. Set aside.
Drain the mussels in a fine sieve or pat them dry with paper towels. Sprinkle some flour over the mussels, ensuring each mussel has a light coating. Shake off the excess flour. In a small bowl, combine the paprika, cumin, and garlic powder. Repeat the first step, but now with the spice mix. Heat the butter in a frying pan and cook the mussels for 3-4 minutes until golden brown. Set the mussels aside.
In another frying pan, heat a tortilla and top it with cheese, spring onions, the cooked mussels, jalapeño, and a bit of lime zest.
Place another tortilla on top and let the cheese melt. Carefully flip the quesadilla and cook until both sides are crispy.
Serve with sour cream, tomato salsa, and guacamole. Enjoy!