Position the rack in the center of the oven and preheat to 375 degrees F.
Place the hazelnuts on a rimmed baking sheet and toast in the oven until fragrant and light golden, 12 to 15 minutes. Remove from the oven and cool. Finely chop nuts and set aside.
Lightly butter a 9-inch-diameter glass or ceramic pie dish. Place 2 1/2 tablespoons butter in a small saucepan and stir over medium heat until melted. Stir in 1/4 cup brown sugar, agave and lemon juice. Pour the butter and sugar mixture into the prepared pie dish. Sprinkle 1/3 cup toasted hazelnuts evenly over the syrup in the dish and set aside. Mix the remaining 1/4 cup brown sugar, cinnamon and nutmeg in a small bowl.
Gently unroll the crescent roll dough on a lightly floured work surface and firmly press and pinch the perforations together to make a solid piece of dough. Roll out the dough to a 12-by-9-inch rectangle using a lightly-floured rolling pin. Sprinkle the brown sugar mixture evenly over the dough to within 1/2-inch of the edges. Sprinkle the remaining 1/3 cup hazelnuts on top.
Starting at one short side, roll up the dough jelly roll-style. Cut the roll crosswise into 8 to 10 rounds, each about 1-inch-wide, using a sharp knife. Arrange the rounds, cut-side down, atop the syrup in the pie dish, with 6 around the edges of the dish and 2 in the center. Pinch open ends together with fingertips to prevent the rolls from unraveling. Cover and chill. Can be prepared 2 hours ahead.
Bake the rolls until deep golden brown on top and the juices are brown and bubbling around the edges of the dish, about 25 minutes. Remove from the oven and let the rolls stand 1 minute. Place a platter atop the pie dish and the invert rolls onto the platter. Sprinkle the rolls lightly with the sea salt flakes. Cool the rolls for 10 to 20 minutes and serve while still warm.