In a small bowl, dissolve yeast in lukewarm milk with 1 tsp of sugar. Let sit for 5–10 minutes until foamy.
In a large bowl, combine flour, remaining sugar, salt, egg, and softened butter. Add the yeast mixture. Knead by hand or with a stand mixer until smooth and elastic (about 7–8 minutes).
Cover with a towel and let rise in a warm spot for 1–1.5 hours, until doubled in size.
Wash, pit, and quarter the plums. Toss with sugar and spices if desired, then set aside.
In a medium bowl, mix flour, sugar, nutmeg and cinnamon. Cut in cold butter with fingertips until the mixture forms coarse crumbs. Refrigerate until ready to use.
Preheat oven to 350°F (175°C). Grease a 9x13 inch pan.
Punch down the risen dough and roll it out to fit the pan. Place in the prepared pan, pressing into corners.
Arrange plum quarters skin-side down in rows over the dough, slightly overlapping.
Sprinkle streusel evenly on top.
Bake for 35–40 minutes, until streusel is golden and dough is baked through.
Cool slightly before slicing.