Bring a large pot of water to a boil. Add 1 tablespoon salt and the green beans and cook until very tender, 6 to 8 minutes. Drain and rinse under cold water to stop the cooking process. Transfer green beans to a bowl.

Whisk together the mustard, vinegar, oil, ¼ teaspoon salt, and ¼ teaspoon pepper. Drizzle over the green beans, toss, and allow to marinate, 30 minutes.

Sprinkle with the toasted pumpkin seeds prior to serving.