How to Make It
Light a grill or preheat a grill pan. Brush the lemon slices with oil and grill over high heat, turning once, until lightly charred, 2 to 3 minutes total. Transfer to a cutting board and let cool. Finely dice the lemon.
Meanwhile, in a large saucepan of salted boiling water, blanch the broccoli rabe until tender, about 3 minutes. Drain and cool under running water; drain again. Transfer the broccoli rabe to a food processor and puree until smooth. Scrape the puree into a measuring cup (you should have about 1 cup).
In a medium saucepan, bring the vegetable stock and 2 cups of water just to a boil; keep warm. In a large saucepan, melt 4 tablespoons of the butter. Add the onion, garlic and a generous pinch of salt and cook over moderate heat, stirring occasionally, until softened, about 5 minutes. Add the rice and cook, stirring, until coated and opaque, about 3 minutes. Add the wine and cook, stirring, until absorbed, about 7 minutes. Add half of the warm vegetable stock and cook over moderately low heat, stirring, until nearly absorbed, about 7 minutes. Add the remaining vegetable stock and cook, stirring, until nearly absorbed and the rice is al dente, 8 to 10 minutes longer.
Stir the broccoli rabe puree into the risotto along with the grated Parmigiano-Reggiano cheese, tarragon, parsley and the remaining 2 tablespoons of butter. Cook, stirring, until the risotto is creamy and heated through, about 3 minutes. Season the risotto with salt and pepper. Spoon into shallow bowls and top with the chopped grilled lemon and dollops of mascarpone.