Heat oven to 450°F. Halve radishes, then toss with olive oil and salt on a rimmed baking sheet. Arrange cut side down and roast 12 to 15 minutes, or until tender and caramelized around the edges. Set aside.

Meanwhile, bring a large pot of salted water to a boil and cook shells to al dente according to package directions. Reserve ½ cup pasta water. Add garlic, mascarpone, sour cream, tahini, lemon juice, and salt to the bowl of a food processor. Process until thoroughly combined. Transfer to a large skillet and warm over medium-low heat (do not boil).

Drain pasta and add to the skillet with the sauce. Let pasta sit for 2 to 5 minutes to allow sauce to thicken slightly.

Add ¼ cup pasta water to the skillet and toss until a creamy sauce forms (add additional water as needed). Add ½ the radishes and ½ the chives and toss again. Divide pasta among 4 shallow bowls and garnish with remaining radishes and chives.