Trim the broccolini by cutting off any dried ends; halve any thick stalks lengthwise so pieces cook evenly.
Bring the water to a boil in a large pot. Add 1 tablespoon kosher salt, then add the broccolini and blanch until bright green and crisp-tender, 2–3 minutes. Drain well and pat dry to minimize splatter.
Heat the olive oil in a large skillet over medium-high heat. Add the garlic and red pepper flakes and cook, stirring, until fragrant but not browned, 30–60 seconds.
Add the broccolini to the skillet and sauté, tossing occasionally, until coated in oil and lightly seared in spots, 2–3 minutes.
Remove from the heat. Add the lemon zest and 1–2 tablespoons lemon juice, then season with 0.5 teaspoon kosher salt and the black pepper. Toss to coat and adjust lemon or salt to taste. Serve hot.