How to Make It
Bring a small pot of water to a boil. Fill a large bowl with ice water. Add 1 cup basil leaves to boiling water; cook for 30 seconds. Drain; plunge basil into ice water and let stand for 1 minute. Remove basil, squeeze out excess liquid, and transfer to a blender.
Add ½ cup oil and ½ teaspoon salt to basil in blender; process until smooth, 15 to 30 seconds. Let stand for 15 minutes. Pour through a fine-mesh strainer into a bowl. Discard solids.
Bring a large pot of generously salted water to a boil over high. Add asparagus; cook until crisp-tender, 2 to 3 minutes. Transfer to a cutting board using tongs. Add tortellini to boiling water and cook according to package directions. Drain; transfer to a large bowl. Stir in lemon zest and half of basil oil (about ¼ cup). Let pasta mixture cool, tossing occasionally, until room temperature, about 30 minutes.
While pasta cools, heat remaining 2 tablespoons oil in a medium skillet over medium-high. Add shallots and ¼ teaspoon salt. Cook, stirring occasionally, until shallots are crisp and golden, 5 to 7 minutes. Transfer to a plate lined with paper towels.
Toss watercress and 1 tablespoon lemon juice in a bowl; arrange on a platter. Chop asparagus into 1-inch pieces and add to pasta in bowl. Add pepper, remaining basil oil, and remaining ¼ teaspoon salt; toss to coat. Spoon over watercress on platter. Drizzle with remaining 1 tablespoon lemon juice; top with shallots and remaining ½ cup basil leaves.
You’ll want to use this easy-prep basil oil on everything this summer. Drizzle it on grilled chicken, veggies, and crusty bread.