Heat oven to 425 F. 

To make the beurre blanc: Medium dice the 2 sticks of butter and store in the refrigerator until needed. 

In a small saucepan over medium heat, combine shallot, white wine and vinegar, stir to combine and bring to a rapid simmer. Reduce liquid until about 2 teaspoons remain. 

Lower the heat to medium-low and add the cold butter 1 tablespoon at a time, whisking until the butter has melted before adding more. Note: The sauce should be the consistency of hollandaise sauce. If it’s too thick, e.g., more like mayonnaise, whisk in 1 tablespoon of water.

Once all of the butter has been added , whisk in the tarragon and season with salt and pepper. Cover and keep warm while you prepare the asparagus and fish. 

To make the asparagus: Toss the asparagus in the olive oil and season with salt and pepper. 

Lay the asparagus flat on a baking sheet and bake for 10 to 15 minutes or until it is tender and beginning to brown in some spots. Set aside.

To make the cod: Using a spice grinder, blend the dried porcini mushrooms into a powder and transfer the powder to a plate. 

Pat the cod fillets dry. Season them with a pinch of salt on one side, then dip that side into the porcini powder, pressing down slightly. Shake off any excess. 

In a large nonstick skillet, heat the grapeseed oil over medium-high heat, place the fillets in porcini-side down, and cook for 3 to 4 minutes, shaking the pan a bit to ensure the fillets don’t stick. 

Flip the fillets over and add the butter. Once the butter has melted and begins to foam, baste it over the fillets for about 3 minutes. Transfer the fillets to a plate lined with paper towels. 

Divide the cod fillets among 4 plates, spoon the beurre blanc over the fillets and set the asparagus alongside them. Serve hot.