1. Melt the butter in a medium saucepan over medium heat. Add the onions, carrots, celery, and bay leaves. Sauté until the onions are translucent, about 3 minutes. Add the flour and mace; cook for another 2 minutes, stirring constantly. Add the chicken stock.
2. Bring to a boil, then reduce to a simmer and continue simmering for 20 minutes.
3. Add the peanut butter and half and half. Cook for 2 more minutes, then remove from the heat and discard the bay leaves. Refrigerate until chilled. Season with salt to taste. Keep chilled until serving.
4. Combine the vinegar and sugar in a medium saucepan. Reduce over low heat until syrupy, about 10 minutes. Set aside.
5. Toss the peach slices in the canola oil. Grill in a cast iron skillet over medium-high heat until lightly caramelized on both sides, about 2 to 3 minutes per side.
6. To serve, divide the soup among serving bowls. Garnish with grilled peach slices, toasted peanuts, and scallions. Finish with a drizzle of vinegar reduction.