Preheat the oven to 425 degrees F and line a baking sheet with aluminum foil.
Bring 8 cups of salted water to a boil in a covered large saute pan. Fill a large bowl with ice water and set a mesh strainer on top of the ice.
Pulse the panko, parsley, the leaves from 1 sprig fresh tarragon and 1/4 teaspoon each salt and pepper in a mini food processor until the herbs are coarsely chopped. Drizzle in 1 tablespoon of the oil and pulse until the mixture is combined. Transfer to a plate.
Combine the brown sugar, mustard, 1 tablespoon of the lemon juice and remaining 1 tablespoon oil in a small bowl. Brush the tops and sides of the salmon generously with the mustard mixture and sprinkle with salt and pepper. Press a salmon fillet, top-side down, into the panko mixture. Place, panko-side up, on the prepared baking sheet. Repeat with the remaining fillets and bake until the panko is toasted and the salmon is just cooked through, 8 to 10 minutes.
Meanwhile, put the asparagus and peas in the saute pan of boiling water and cook until almost crisp-tender, about 3 minutes. Transfer the vegetables to the prepared ice bath with a slotted spoon and let cool, about 1 minute.
Meanwhile, heat a second large saute pan over medium-high heat and add the butter, swirling, until melted. Drain the cooled vegetables and add to the pan. Season with salt and pepper. Cook, stirring occasionally, until tender, 3 to 5 minutes. Remove from the heat, add the radishes, the remaining 2 tablespoons lemon juice and 2 tablespoons tarragon. Toss to combine, taste and adjust the seasoning. Serve the salmon with the asparagus salad and a lemon wedge.
If your radishes have beautiful greens attached, give them a wash, roughly chop and saute them with the asparagus and peas.
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