MAKE THE TARRAGON-CHIVE AIOLI: Using a whisk and a large bowl, or a blender, or a food processor, blend the yolks, lemon juice, a pinch of kosher salt, and mustard powder. Add the ¼ cup each chives and tarragon. (If making by hand, mince the fresh herbs before adding to the egg mixture.) Drizzle in the oil very slowly at first whisking or pulsing continually until emulsified, smooth, and flecked with the finely minced herbs. Don’t overmix. Taste and adjust seasonings. If too bitter, whisk in a ½ teaspoon honey. Set aside or cover and refrigerate up to 2 days ahead.
POACH THE SALMON: In a 4-quart dutch oven or medium lidded skillet, bring 4 cups water to boil with the reserved tarragon sprigs, 2 tablespoons kosher salt, smashed garlic, and 2 teaspoons peppercorns. Add salmon, cover; remove from heat. Let stand until salmon is cooked (opaque, but still juicy) about 6 minutes. Begin checking for doneness at 5 minutes.
SAUTÉ THE ASPARAGUS: While the water for the salmon heats and the salmon poaches, heat a small skillet over medium-high flame. Add 2 tablespoons olive oil and heat for another minute until shimmering. Add the asparagus -- it’ll seem all piled up – and season with 1/4 teaspoon of kosher salt, a few grinds of fresh black pepper, the pinch of crushed red pepper, and the lemon zest; toss well. Sauté, tossing every minute or so with tongs, until just crispy tender. Taste and adjust seasonings. Remove from heat and set aside loosely covered if the salmon is not yet done.
SERVE HOT OR WARM OR AT ROOM TEMPERATURE OR COLD: Add a salmon fillet to each of 2 warm plates or shallow bowls (if serving hot or warm) with some asparagus to the side. Ladle the aioli over the salmon. Garnish with reserved minced chives, a sprinkle of flaky salt, and a grind or two of fresh pepper.