1
: Preheat broiler.
2
: Heat oil in a 10” (25cm) or 12” (30cm) nonstick ovenproof skillet over medium heat. Add onion and garlic. Cook 3-5 minutes until tender. Turn heat down if onions and garlic start to brown. Add spinach and watercress and cook until wilted, about 1-2 minutes. Transfer cooked mixture to a plate. Return pan to heat.
3
: Whisk eggs and evaporated milk in bowl. Add mixture to hot pan. Allow eggs to cook for 2-3 minutes until bottom starts to set. Scatter cooked spinach mixture and goat cheese over eggs. Continue cooking until eggs are almost set. About 3-4 minutes.
4
: Transfer pan to oven. Broil until center is set and cheese starts to melt, about 2-3 minutes. Using spatula, loosen edges of frittata and slide out onto platter. Serve warm or room temperature.