Add the onion and garlic to the pot and cook over medium-high heat, stirring occasionally, for about 5 minutes, until golden and tender. Add the cinnamon, oregano, cumin, coriander, and crushed red pepper. Continue to cook, stirring, 1-2 minutes, until fragrant. Add the mashed tomatoes, and cook, stirring occasionally, for 10 minutes. Add the diced tomatoes and 2 cups of water. Bring to a boil, then turn the heat to low and simmer, stirring once or twice, for 15 minutes. Transfer the cooked ground lamb back into the pot and stir to combine. Cover and cook, stirring occasionally, for 20 minutes. Fold in the spinach and season with salt and pepper to taste.