1. Preheat oven to 425°F (220°C). Trim top of garlic head to expose cloves. Arrange, cut side up, in centre of 10-inch (25 cm) square of foil; drizzle with oil and sprinkle with pinch salt. Fold foil over and seal edges to make packet. Place on baking sheet. Bake until tender and golden, about 45 minutes. Unwrap; let cool 15 minutes. 2. Meanwhile, pulse frozen greens in food processor until finely chopped. Scrape into fine-mesh strainer set over large bowl. Set aside.
3. Wipe out food processor. Squeeze garlic cloves into food