Prepare the poaching syrup: In a saucepan large enough to hold the peaches in a single layer, combine water, 1 cup granulated sugar, and 1 lemon's juice. Bring to a simmer over medium heat, stirring until the sugar dissolves.

Prepare the peaches: Cut the peaches in half and remove the pits. Gently place the peach halves into the simmering syrup.

Poach the peaches: Reduce heat to low, cover, and simmer gently for 15-20 minutes, or until the peaches are tender but still hold their shape. Carefully remove peaches from the syrup and set aside to cool.

Make the raspberry sauce: In a small saucepan, combine raspberries, 1/4 cup granulated sugar, and 1 tablespoon lemon juice. Cook over medium heat, stirring occasionally, until the raspberries break down and the sauce slightly thickens, about 5-7 minutes. Press the sauce through a fine-mesh sieve to remove seeds.

Assemble the dessert: Place two peach halves in each serving bowl. Add a scoop of vanilla ice cream to the center of the peaches.

Drizzle generously with the raspberry sauce and sprinkle with toasted slivered almonds. Serve immediately.