If you're making homemade pesto, start with that. Prepare haloumi: cut the cheese block into 1 cm slices.
Heat a non stick skillet over medium hight heat. Add pine nuts and toast until fragrant (make sure you don't burn them!). Remove from skillet and set aside.
In the same skillet, grill haloumi slices about one minute on each side or until golden brown. Set aside. When haloumi is cool enough to handle, cut it into cubes.
Prepare the pesto ricotta. In a small bowl, combine pesto and ricotta.
Arrange mushroom caps upside down in a baking dish. Stuff them with 2 to 3 tablespoons of pesto ricotta mixture (depending on the size of the caps). Top with haloumi cubes and sprinkle generously with pine nuts. Bake for 20 minutes and serve warm on a bed of salad.