Place the potatoes in a large saucepan and cover with water by 1 inch. Add 1/2 teaspoon of salt and bring to a boil. Reduce the heat to medium-low and simmer until fork tender, about 10 minutes. Drain in a colander.
Return the potatoes to the saucepan. Add 4 ounces (1 stick) of the butter, the cream, 1/2 teaspoon salt, and 1/4 teaspoon black pepper. With the heat on medium-low, mash and stir the mixture until well blended, about 4 to 5 minutes. Adjust seasoning, to taste, and cover to keep warm.
Bring a medium pot of salted water to a boil. Add the fiddlehead ferns and blanch until just tender, about 1 1/2 minutes. Remove with a slotted spoon and shock in an ice bath. Drain on paper towels. Add the ramps to the boiling water and blanch until just tender, about 2 minutes. Drain in a colander and shock in an ice bath. Drain on paper towels and set aside.
Heat the olive oil and 2 tablespoons of the butter in a large saute pan over a medium-high heat. Add the morels and cook, stirring, for 3 minutes. Add the shallots and garlic. Season with the salt and pepper, and cook until the mushroom liquid has been released, about 4 minutes. Add the stock, and bring to a boil. Cook, stirring occasionally until the volume is reduced by half, about 3 minutes. Add the crawfish tails and stir well to mix. Lower the heat to medium-low, and gradually add 6 ounces (1 1/2 sticks) of butter several pieces at a time, stirring constantly to incorporate. Add the fiddlehead ferns and toss until they are heated through. Remove from the heat and cover to keep warm.
Season the salmon on both sides with 1/4 teaspoon of salt and 1/4 teaspoon of pepper per fillet. In a large skillet or 2 smaller skillets, melt 2 tablespoons of butter over medium-high heat. Add the salmon and sear for 2 to 3 minutes per side for medium-rare. Remove from the heat.
Melt the remaining 2 tablespoons of butter in a medium skillet over medium-high heat. Add the pancetta and cook until golden brown and some of the fat is rendered. Add the shallots and saute for 1 minute. Add the ramps, and salt and pepper to taste, and cook, stirring, until tender yet still firm, 3 to 4 minutes.
Divide the morel ragout among 6 dinner plates. Place a dollop of mashed potatoes in the middle of the plate and top with a salmon fillet. Spoon the ramps on top of the fish and serve immediately.