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.
Bring to a boil, then reduce to a simmer and continue simmering for 20 minutes.
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.
Combine the vinegar and sugar in a medium saucepan. Reduce over low heat until syrupy, about 10 minutes. Set aside.
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.
To serve, divide the soup among serving bowls. Garnish with grilled peach slices, toasted peanuts, and scallions. Finish with a drizzle of vinegar reduction.