Pizza with Spring Onions and Watercress

Name: Pizza with Spring Onions and Watercress
Ingredients: 2 Tbs. unsalted butter; 2 Tbs. all-purpose flour; 1 1/4 cups (10 fl. oz./310 ml) plus 1 Tbs. milk; 1 garlic clove, minced; 1/2 cup (2 oz./60 g) grated Parmesan cheese; Kosher salt and freshly ground pepper; 3 Tbs. extra-virgin olive oil; 4 spring onions, halved lengthwise; All-purpose flour for dusting; 1 batch food processor pizza dough; 1/2 cup (2 oz./60 g) grated mozzarella cheese; 1/2 cup (2 oz./60 g) grated Parmesan cheese, plus more as needed; Extra-virgin olive oil for drizzling; 1 bunch watercress, coarse stems removed; Flaky sea salt