First, make the fondant. It needs to be refrigerated overnight before using. To make the fondant, you'll need a marble slab or a baking sheet and an offset spatula to fold the candy. If you're using a baking sheet, rinse it with water, leaving a few drops on the pan.
Dissolve the cream of tarter in the water in a heavy-bottomed saucepan. Add the sugar and bring the mixture to a boil over medium-high heat. Swirl the pan occasionally to help dissolve the sugar. Do not stir once the sugar syrup begins to boil. Heat to 240 degrees F (115 degrees C), or until a small amount of syrup dropped into cold water forms a soft ball that flattens when removed from the water and placed on a flat surface.
Immediately pour the candy onto the baking sheet or marble slab and let cool for 10 minutes. Use a spatula to vigorously knead the candy, folding it over itself. Knead until it begins to look cloudy, 5 to 10 minutes; allow it to cool. Transfer the fondant to a heatproof bowl or the top of a double boiler. Refrigerate overnight.
To make the sponge, dissolve the yeast and 1 teaspoon sugar in 1/4 cup of the room-temperature milk. Pour the remaining 1/2 cup milk into the bowl of a stand mixer. Add 1 cup of flour and the yeast mixture. Mix with a fork to combine. Cover the bowl with a towel and allow to rest until bubbly, 10 to 15 minutes.
Stir the remaining 3 cups of flour, 1/4 cup sugar, salt, room-temperature eggs, and zest from 1 lemon into the sponge. Use the dough hook to mix the dough on the lowest setting. Mix the dough for about 5 minutes, occasionally scraping the dough off of the hook and down the sides of the bowl.
Increase the speed to medium-low and add the softened butter 1 tablespoon at a time until fully incorporated. Allow the dough to mix for an additional 5 minutes, scraping the dough down as needed. Add the ground almonds and turn the mixer on for 1 or 2 turns, mixing just until incorporated. Transfer the dough to a lightly floured work surface and shape it into a ball. Place the ball in an oiled mixing bowl and cover with plastic wrap and a kitchen towel and let it rise until doubled, 1 1/2 to 2 hours.
While the dough is rising, prepare the filling. Heat the milk in a heavy 4-quart saucepan until it begins to boil; stir in the butter and sugar. Remove the pan from the heat and add the poppyseeds, 1 1/2 teaspoons of lemon zest, and the cinnamon. Let the mixture stand for 30 minutes to soften the poppyseeds. When the mixture is cool, whisk in the egg.
Punch down the dough and roll it out into a 10 1/2- by 16-inch rectangle. Spread the filling evenly over the dough, leaving a 1-inch margin on all sides. Roll the dough up into a log, starting with the long side, and pinch the edges together to form a seam. Place the loaf, seam-side down, on a parchment-lined baking sheet. Cover it with greased plastic wrap or a well-floured kitchen towel and allow it to rise for an additional 30 to 40 minutes.
Preheat an oven to 400 degrees F (200 degrees C).
Bake the stollen until golden brown on the bottom of the loaf, about 40 minutes. Let the bread cool completely before glazing.
To glaze the bread, add the lemon juice to the fondant. Place the bowl over a pan of simmering water, stirring frequently with a rubber spatula until soft and glossy. Pour the fondant over the stollen and allow it to cool before slicing.