How to Make It
Whisk together sour cream, whole-grain mustard, Dijon mustard, and 1 teaspoon lemon juice in a small bowl. Spread mixture evenly on one side of toast slices. Toss together mustard greens and remaining 1 teaspoon lemon juice in a separate bowl; season to taste with salt and pepper. Mound greens evenly on toasts.
Heat oil in a large nonstick skillet over medium. Crack eggs into skillet, spacing at least 1 inch apart; cook until whites just start to change color, about 30 seconds. Add 1/3 cup hot water to skillet; cook, basting eggs with water, until whites are just set and yolks are still runny, about 2 minutes. Place 1 egg on each prepared toast. Season with salt and pepper to taste. Serve immediately.
Creamed mustard can be made and stored in an airtight container in refrigerator up to 5 days ahead.