To prepare the tagine, combine first 6 ingredients in a spice or coffee grinder; process until finely ground.
Heat oil in a large Dutch oven over medium heat. Add onion, carrot, celery, 1/2 teaspoon salt, and 2 peeled garlic cloves; cook 5 minutes, stirring occasionally. Cover, reduce heat to low, and cook 20 minutes.
Stir in cumin mixture, sweet potato, and next 5 ingredients (sweet potato through tomato); bring to a boil. Reduce heat; simmer, uncovered, 30 minutes or until thick. Stir in lemon juice.
To prepare the tempeh, preheat oven to 350°.
Combine 2/3 cup water and next 7 ingredients (2/3 cup water through 4 minced garlic cloves) in a large bowl. Add tempeh, and toss well to coat. Arrange the tempeh mixture in a single layer in an 11 x 7-inch baking dish. Cover with foil.
Bake at 350° for 35 minutes. Uncover and bake an additional 5 minutes or until liquid is absorbed. Serve tempeh over tagine and couscous; sprinkle with cilantro, if desired.