Put morels, porcinis or both in a bowl with very hot water to cover; soak until soft, about 20 minutes. Drain morels
and reserve soaking liquid. Cut morels in half; if porcinis are large, chop
butter in a large skillet over medium-high heat; when butter
is hot and foam
has subsided, add shallots and reconstituted and fresh mushrooms to pan. Cook until shallots soften and fresh mushrooms have released their liquid and it has cooked off, about 10 minutes.
Add asparagus and 1/2 cup reserved liquid to pan. Bring liquid to a boil, cover, reduce heat so mixture simmers, and continue cooking for another 2 to 4 minutes, or until asparagus is crisp-tender. Add cream and tarragon
and continue cooking, uncovered, until sauce
thickens slightly and asparagus is tender, about 4 minutes more. Season with salt and pepper and serve.