Heat 2 tbsp. of the olive oil in a Dutch oven over medium-high heat. Add the leek, onion, and garlic. Stir and cook for 3 minutes, or until tender. Add the squash, potatoes, and 1 tsp. of the salt. Stir and cook for 5 minutes. Add the stock, cover, and simmer for 20 minutes, or until vegetables are tender.
Transfer the soup to a blender and process on medium until smooth, about 1 minute. Be careful, as hot liquids expand, so you may need to process in batches. Add more stock if you prefer a thinner consistency.
Toss together the greens with the remaining 1 tbsp. olive oil and 1⁄4 tsp. salt. Sprinkle soup with pepitas. Top cheese toast with the greens and serve on the side.