Pre-heat the oven to 375°F and lightly grease a casserole dish.
Wash, trim and simmer or steam the asparagus spears, in batches if necessary, for 4-5 minutes depending on the thickness of the spears.
Drain well and arrange in the casserole dish.
Sprinkle the salt, pepper and garlic over the asparagus, then stir the cornstarch into the sour cream, pour it over the asparagus and spread to the edges of the dish.
Sprinkle the breadcrumbs and tarragon over the sour cream and place small pieces of butter over the breadcrumbs.
Bake for 15-20 minutes.