Preheat the oven to 425°F with the wire rack placed in the center of the oven.
Pat the dry chicken with paper towels and place on a foil lined baking sheet.
In a small bowl, stir together 2 tablespoons of olive oil, red curry paste and mandarin orange zest until well combined.
Gently rub the red curry paste mixture between the skin and flesh of the chicken thighs, as well on the skin.
Season the chicken generously with salt and pepper.
In a small bowl, toss the mandarin orange slices, onion, fennel and garlic in the remaining tablespoon of olive oil. Season to taste with salt and pepper and scatter on the baking sheet with the chicken.
Roast in the oven for 35 minutes, or until the chicken is 165°F.
Garnish with basil and fennel fronds. Serve immediately.