Preheat the oven to 350 degrees . In a small saucepan, cook the olive oil and garlic cloves over low heat until tender, about 30 minutes. Strain, reserving the garlic oil.
Using a blender, puree the garlic cloves; reserve 1/2 tablespoon. With the machine on, add the lemon juice and 3 tablespoons reserved garlic oil until combined; season with salt.
Using a food processor, pulse the whole hazelnuts until coarsely ground. With the machine on, pulse in 2 tablespoons reserved garlic oil, the orange peel and orange juice, the reserved 1/2 tablespoon garlic puree, 1 1/4 teaspoons salt, 1 teaspoon lemon peel and the sugar; add more garlic oil to thin the mixture to a spreadable consistency as necessary.
Arrange the bread slices on a baking sheet and brush with garlic oil; toast until golden, 10 to 15 minutes. Transfer to a work surface. Spread each toast with one-quarter of the hazelnut mixture and 1/4 cup ricotta. Sprinkle the remaining lemon peel and the garlic-lemon vinaigrette on the ricotta and top with the chopped hazelnuts and parsley.