1/2 small red onion peeled, chopped into 1-inch squares
1 tbsp white vinegar
1 egg yolk
1/4 tsp kosher salt
1 cup vegetable oil
2-3 tsp fresh lemon juice
2-3 tsp warm water
1 tsp parsley
Preheat oven to 350 degrees. Toss the onion with the vinegar and place it on a baking sheet. Bake for 20 min, allow to cool, and finely chop.
In a medium sized bowl whisk the egg yolk and salt by hand. Drizzle in the oil drop-by-drop whisking constantly. Add a teaspoon each of the lemon juice and water as it the mixture thickens.
Once half the oil is added, use a mixer with a whisk attachment on medium speed to whisk the mixture while you steadily add the oil in a slow trickle until done. Add lemon juice and water as it thickens.
Stir in the onions and parsley. Season with lemon juice and salt to taste.