Combine the boiled potatoes and milk in a large mixing bowl; using a potato masher, mash the potatoes until smooth and creamy. Set aside.
Heat 1 tablespoon butter and 1 tablespoon olive oil in a large skillet over medium heat.
Add chopped onions and cook for 2 to 3 minutes, stirring occasionally, until translucent.
Add spinach and garlic to the skillet; season with salt and pepper and continue to cook for 2 more minutes or until spinach is wilted.
Remove from heat and add spinach mixture to the potatoes. Stir in cheese and lemon juice. Add the egg and panko/bread crumbs; stir until thoroughly combined and smooth.
Shape the mixture into patties.
Heat vegetable oil in a large skillet.
Working in batches, fry each potato patty on both sides until golden brown, about 3 to 4 minutes per side.
Transfer the potato patties to a paper towel–lined plate while working with the rest.
Serve the potato patties with yogurt, sour cream, soy sauce, etc...