NOTE: *If you own an egg-poaching pan, use it to prepare the eggs.
In a small microwave-safe container, combine the mustard, vinegar, and olive oil. Microwave 30 seconds or until the mixture boils. Set aside.
In the meantime, bring a large pot of water to boiling. Break each egg carefully into a measuring cup and gently slide the egg into the water. Repeat with remaining eggs.
Cover pan and simmer eggs approximately 6-9 minutes or until the whites are completely set and the yolks thicken but are not completely hard.
Meanwhile, in another small skillet, heat the butter and lightly wilt the mixed greens or spinach for no more than 15 seconds. Remove from heat and set aside.
Arrange the toasted rye bread onto 4 salad plates. Next, arrange the sauteed salad greens or baby spinach onto each plate. Top each serving with a poached egg. Finally, drizzle with the mustard dressing.