4 spring onions, thinly sliced, green and white parts separated
1 tsp. minced garlic
2 Tbs. dry white wine
2 Tbs. chopped flat-leaf parsley
1 tsp. whole grain mustard
2 tsp. drained capers
1 Tbs. champagne vinegar
1/2 cup olive oil
Kosher salt and freshly ground pepper to taste
5 oz. arugula
Combine the white parts of the onions, the garlic, and white wine in a small saucepan over low heat and cook until the wine is reduced by half, about three minutes. Set aside to cool. Once cool, combine the mixture, the parsley, mustard, capers, and vinegar in a blender and blend until smooth. With the motor running, slowly stream in the olive oil. Add salt and pepper to taste.
Combine the arugula and green parts of the onions in a serving bowl and toss with the dressing. Place on top of each wedge of quiche.