Whisk together first 3 ingredients, 1/2 teaspoon salt, and 1/2 teaspoon pepper. Set aside to chill.
Preheat oven to 325°F. Place potatoes on baking sheet; add 1 tablespoon oil and 3/4 teaspoon salt, and toss to coat.
Bake in preheated oven, about 30 minutes, stirring once halfway through.
Heat a large cast-iron skillet over medium. Add 1 tablespoon oil; swirl to coat. Add onion and 1/2 teaspoon salt, and cook, stirring, about 5 minutes. Add bell pepper, and cook, stirring, 5 minutes. Remove onion and bell pepper.
Increase heat to medium-high. Add remaining 3 tablespoons oil to skillet. Add potatoes; cook until golden brown, 4 minutes. Stir and cook 2 more minutes. Add bell pepper mixture, 3/4 teaspoon salt, and 1/4 teaspoon pepper, and cook 1 minute. Remove from heat; stir in chopped parsley. Serve with crème fraîche mixture.