In a blender, combine flour, sugar, salt, and pepper, then add room temperature eggs and milk. Blend until completely smooth—this ensures no lumps develop. With the blender running on low speed, slowly drizzle in the cooled melted butter until fully incorporated. Let the batter rest at room temperature for at least 15 minutes before cooking; this allows the flour to fully hydrate and makes the crepes more tender.

Heat a 9-inch nonstick skillet or crepe pan over medium heat for about 3-5 minutes until evenly hot. Lightly oil the pan with a brush or paper towel dipped in oil. Pour about 1/4 cup batter into the center, then immediately tilt and swirl the pan to spread the batter into a thin, even circle. Cook for about 45 seconds until the bottom is light golden, then flip carefully and cook the other side for about 20 seconds. Transfer to a plate and repeat with remaining batter. I find stacking them between parchment paper keeps them from sticking together. You should get 10-12 crepes depending on size.

Heat 1 tablespoon butter with 1 tablespoon oil in a large skillet over medium-high heat. Add the sliced mushrooms in a single layer (don't overcrowd) and let them cook undisturbed for 3-4 minutes until golden brown on one side, then stir and cook another 2-3 minutes until tender and browned. Transfer to a bowl and season with salt and pepper. In the same skillet, add the remaining 1 tablespoon butter and 1 tablespoon oil, then add asparagus pieces and sauté for 4-5 minutes until bright green and just tender-crisp. Season with salt and pepper, then combine with the mushrooms in the bowl.

In a small pot, heat 1 cup milk, 1/2 cup vegetable stock, and minced garlic over medium heat until small bubbles begin to form around the edges (don't boil). In a separate medium pot, melt 2 tablespoons butter over medium heat and add diced onion, cooking for 4-5 minutes until softened and translucent. Sprinkle the flour over the onions and stir constantly for 2-3 minutes to create a paste—this cooks out the raw flour taste. Remove the garlic from the milk mixture, then slowly whisk the warm milk mixture into the onion-flour paste while stirring to avoid lumps. Simmer gently for 7-10 minutes, stirring occasionally, until the sauce thickens enough to coat the back of a spoon.

Remove the sauce from heat and stir in the Dijon mustard, lemon zest, truffle oil, fresh herbs, ground nutmeg, and a pinch each of salt and pepper. Taste and adjust seasonings as needed. I prefer to add the delicate flavors at the end so they stay bright and don't get cooked out. Reserve about 1/3 of the sauce for serving alongside the crepes, and fold the sautéed mushrooms and asparagus from Step 3 into the remaining sauce.

Warm the cooked crepes briefly in a 300°F oven if they've cooled. Lay a crepe on a plate and spoon about 3-4 tablespoons of the vegetable-filled sauce down the center. Roll the crepe tightly and place seam-side down on a serving plate. Repeat with remaining crepes and filling. Drizzle the reserved plain sauce around the crepes and garnish with additional fresh herbs if desired. Serve immediately while warm.