In a medium pot, add the potatoes, some salt and water to cover. Bring to a boil. Cook the potatoes until just tender, 12 to 15 minutes. Strain and halve the potatoes.
In a small glass jar, combine the oil, mustard, vinegar, agave syrup and garlic. Shake vigorously until emulsified. Taste, and season with salt and pepper.
To assemble: Combine the potatoes and red peppers in a large mixing bowl. Toss with the vinaigrette. Season with salt and pepper. Garnish with the scallions, if using.