In a food processor, combine garlic and parsley. Pulse to combine until parsley leaves are well broken down. Slowly drizzle in ½ cup of olive oil until all is incorporated. Scrape down sides and season to taste with salt and pepper.
In a medium-sized pot over high heat, bring water to a boil. When a rolling boil forms, add pasta and cook to al dente, according to package directions (approximately four to eight minutes). Remove from heat, drain, and rinse in colander with cold water.
In a small skillet over medium heat, toast slivered almonds, stirring constantly until fragrant and browned slightly, about 4 minutes.
Combine pasta, asparagus, peas, and scallion in a large bowl. Toss gently with pistou and top with slivered almonds.