Make the fennel syrup: Coarsley grind fennel seeds in a spice grinder. Add seeds, sugar and water to a small saucepan set over medium heat. Bring mixture to a boil and cook, stirring occasionally, until mixture begins to thicken, about 5 minutes. Remove from heat, cover and let stand for 20 minutes. Pour the fennel syrup through a fine strainer and let cool until room temperature, about 30 minutes. Syrup will keep, airtight, in the regrigerator for up to 1 month. Make the spritzer: Stir together 2 tablespoons fennel syrup, apple juice, and lemon juice in a tall ice-filled glass. Top with seltzer, and garnish with apple slices and fennel fronds, if using.