Embellished with edible flowers, this springtime salad will rise to a special occasion--in May, it's perfect for a Mother's Day brunch.
Squeeze 2 lemon halves into a large bowl of water; add rinds. Working with 1 artichoke at a time, remove tough outer leaves closest to stem end, then trim pointy tips of remaining outer leaves with kitchen shears. With a serrated knife, cut off top 1 inch; with a paring knife, trim and peel stem. (Transfer to lemon water as you work to prevent discoloration.)
Set a steamer basket in a large pot; add enough water to reach just below basket and bring to a boil. Squeeze remaining 2 lemon halves into pot. Add artichokes, season with salt, and cover; steam 20 minutes. Add potatoes to pot. Continue to steam until potatoes and artichoke hearts are easily pierced with the tip of a knife, 12 to 15 minutes. Transfer to a plate; let stand until cool enough to handle.
Meanwhile, prepare an ice-water bath. Bring 2 inches of water to boil in a saucepan; season with salt. Add asparagus and boil until crisp-tender, about 1 minute. Transfer to ice bath and let cool. Drain; pat dry.
Remove leaves from artichokes; reserve for eating separately. Scoop out and discard fuzzy chokes, then halve artichoke bottoms lengthwise. Cut potatoes into 1/4-inch rounds. Divide artichoke bottoms, potatoes, asparagus, greens, and radishes among 4 plates.
Whisk together mustard and vinegar in a bowl. Gradually whisk in oil; season with salt and pepper. Lightly drizzle salads with some of dressing; garnish with flowers. (Remaining dressing can be served separately, with reserved leaves.)