Preheat the oven to 375°F (190°C).
In a large pot filled with water, boil the potatoes with ½ tablespoon of salt until tender, 15-20 minutes.
Meanwhile, in a small saucepan, bring 2 cups of water to a boil. Remove from heat and add the spinach for 2 minutes. Strain the hot water, run the spinach under cold water and then squeeze out the excess water. Place the spinach into a blender and add the milk. Blend until smooth.
Once the potatoes are cooked, strain. Using a potato masher, mash the hot potatoes with butter and spinach mixture until nice and smooth. (Make sure to mash the potatoes as soon as you strain for the fluffiest texture.) Taste it and salt and pepper to taste. Transfer it into a pastry bag, or a ziplock bag with one end cut.
While potatoes are cooking, in a medium bowl, mix together ground beef, egg, garlic, Worcestershire sauce, salt and thyme. (Dig in with your clean hands; it’s the best way.) Add the frozen peas and mix well.
Divide the meat mixture into 12 muffin cups. Pipe the mashed potatoes on top. (As meat shrinks during cooking, pipe the mashed potatoes so that it doesn’t completely cover the meat.)
Bake for 25-30 minutes and serve warm.