Serves 2; can be doubled, tripled, etc.
8 baby Yukon Gold potatoes, cut in half
1/4 large red onion, cut into 1-inch pieces
1/2 fennel bulb, trimmed, cut into 1-inch pieces (reserve the fronds)
2 Tbsp balsamic vinegar
2 Tbsp agave nectar
1-1/2 tsp Dijon mustard
1/4 cup extra-virgin olive oil (not fruity), plus 1-2 tsp for roasting
1/4 tsp kosher salt
1/8 tsp fresh black pepper
Preheat oven to 425°F.
Place potatoes, onion, and fennel on a rimmed baking sheet. Drizzle with 1-2 teaspoons of olive oil (depending on the amount of vegetables you're making), and season with kosher salt and black pepper. Toss, with your hands, to be sure all of the vegetables are coated. Roast for 30-40 minutes, turning the vegetables just one time, until the potatoes are cooked through and starting to brown slightly. Remove from the oven and transfer to a mixing bowl.
While the potatoes are cooking, combine remaining ingredients in a jar with a tight-fitting lid, and shake the jar to emulsify the dressing.
Spoon as much of the dressing as you need onto the warm vegetables, and stir to combine. The vinaigrette will absorb into the warm vegetables. Set aside to cool to room temperature. Garnish with the reserved fennel fronds, and serve. (You can make this several hours in advance.)