1) Melt 15g of the butter in a large skillet, over medium-high heat, with the cumin, coriander, and toast the spices until fragrant, about 30 secs. Add the beans and mash with a potato masher into a thick paste. Mix in pinch of salt and transfer to a bowl. Rinse and dry the skillet.
2) Lay out half the tortillas on a work surface. Spoon and spread the beans evenly onto the tortillas. Scatter the cheese evenly over the beans, and top with the remaining tortillas.
3) Halve, seed and dice the avocado. Toss with the tomato and season with salt and pepper, to taste.
4) Melt about 10g of the butter over medium heat in the skillet. Cook the quesadillas one at a time, until golden and the cheese melts, about 1.5 to 2 minutes per side. (Add a bit more butter, if needed when they are turned.) Transfer to a cutting board and cool briefly before cutting into wedges. Serve warm with the avocado mixture and the sour cream, if desired.