1. Preheat a pan over medium-high heat and coat with cooking spray.
2. Spread 1 tablespoon of the quince paste on each of 4 bread slices. Top with 1/4 of the cheese and 1/4 of the toasted walnuts. Cover with remaining bread slice, spray both sides of the sandwich with cooking spray, and toast each of the sandwiches in the pan, flipping once, until both sides are lightly toasted and the cheese is melted, about 8 minutes total.