Preparation

• Trim the woody ends from the asparagus and put them in a 4-quart saucepan. Add the quartered onion, 1 tsp. salt, and 8 cups of water. Bring to a boil, then turn the heat down and simmer until the broth is reduced to 5 cups, 25 to 30 minutes.
• Meanwhile, separate the asparagus tips from the stalks. Halve each tip lengthwise and cut the stalks crosswise 1/2 inch thick; set aside. In a small bowl, whisk the cream and cornstarch.
• Strain the broth into a bowl. Wipe out the saucepan.
• Melt the butter in the saucepan over medium-high heat, add the chopped onion, and cook, stirring occasionally, until beginning to brown, 6 to 10 minutes. Add the garlic and mustard and cook, stirring, for 30 seconds. Add the wine and cook, stirring, until almost evaporated, about 1 minute. Add the broth, potatoes, and 1 tsp. salt. Bring to a boil, then turn the heat down and simmer until the potatoes are tender when pierced with a fork, 6 to 9 minutes. Using a potato masher or fork, gently mash about half of the potatoes against the sides and bottom of the saucepan.
• Whisk the mascarpone into the chowder and bring to a bare simmer over medium-low heat. Whisk in the cornstarch mixture and continue to gently simmer, whisking, until the chowder is slightly thickened, about 2 minutes. Add the asparagus tips and stalks, and cook until bright green and just tender, 2 to 3 minutes. Remove from the heat and stir in the chopped dill, 1 tsp. salt, and 1 tsp. pepper. Season to taste with more salt and pepper, and serve garnished with the dill fronds and a few drops of hot sauce, if using.