Heat a small skillet over low heat. Add sesame seeds and toast, stirring almost constantly, until golden brown, about 2 minutes. Add coriander and cumin; cook, stirring until fragrant, 10 to 20 seconds longer. Turn out onto a plate and let cool. Grind with a mortar and pestle or in a spice mill and set aside. Add oil to the skillet and heat over medium heat. Add scallions and garlic; sauté until softened, about 2 minutes. Set aside.
Prepare a grill or preheat the broiler. In a medium-sized bowl, mash chickpeas coarsely with a potato masher. Stir in rice, 1/3 cup wheat germ, lemon juice, salt, pepper, the reserved sesame seed-spice mixture and the scallion-garlic mixture; mix well. Shape the mixture into four 3/4-inch-thick patties. In a shallow dish, dredge the patties in the remaining 1/3 cup wheat germ, pressing the wheat germ firmly onto the patties.
Grill or broil the patties and tomato slices on a lightly oiled rack until browned and heated through, about 3 minutes per side. Tuck the patties into pitas; garnish with yogurt and spinach.