1. Heat a large skillet over medium-high heat. Add beef and garlic; cook until beef is browned, breaking into small pieces with the side of a large spoon.
2. While meat cooks, peel and cube sweet potatoes.
3. When beef is half cooked, drain off any accumulated fat. Add sweet potatoes and sesame seeds; continue to cook.
4. Meanwhile, rinse spinach well, remove any heavy stems, and coarsely chop.
5. When beef is well browned and sweet potato cubes are starting to soften, about 10 minutes, combine water, soy sauce, and cornstarch until cornstarch dissolves. Pour over contents in skillet. Simmer until sweet potato cubes are soft but not mushy, another 10 to 12 minutes.
6. Just before serving, add spinach and cook 2 to 3 more minutes until spinach is wilted, stirring once. Serve over hot rice.