Step 1

Heat the oven to 400°F. Use 1 tablespoon of the oil to grease a large rimmed baking pan (or “half-sheet,” 18- by 13-inches). Measure the ground spices into a small bowl, add a pinch of salt and black pepper to taste and stir. Make sure the vegetables are all ready to go before starting.

Step 2

Put 1 tablespoon of the remaining oil in a large skillet over medium-high heat. When it’s hot, add the chicken, either side down, and sprinkle with some salt. Cook until the pieces are browned and release easily from the pan with a spatula, 3 to 5 minutes; flip, sprinkle this side with a little more salt and cook the same way. (It will still be pink inside.) Transfer the chicken to a plate.

Step 3

Turn the heat to medium, add 11/2 cups water and the teff to the skillet, scraping up any browned bits and stirring with the spatula. Adjust the heat so the mixture bubbles gently and cook, stirring constantly, about 3 minutes. Spread the teff mixture into the bottom of the prepared baking pan. Shake the pan to distribute the  teff to each corner; it’s okay if the layer of grains is thinner in some places than others.

Step 4

Rinse out the skillet and return to medium heat. Add the remaining 2 tablespoons oil and when it’s hot, scatter in the onions and garlic. Cook, stirring occasionally until they’re soft and starting to color, 3 to 5 min. Add the spice mixture and stir until fragrant, less than a minute. Add the preserved (or fresh) lemon, the olives, and 1 cup water. Bring to a boil, stirring once or twice, then remove from the heat. Taste and adjust the seasoning; the mixture should be salty enough to season the vegetables and teff.

Step 5

Nestle the chicken, tomatoes, and green beans into the teff. Spoon the seasoning mixture from the skillet over all; use a soft spatula to scrape every drop into the sheet pan. To tent-and-vent the pan to mimic how a tagine cooks, join two 16-inch sheets of foil by stacking them and closing one long edge with two small folds. Cover the pan tightly around the edges so the seam is crosswise across the center and open a hole in the fold that’s about 5 inches long. Pull up on the seam so the hole looks like an open mouth above the pan.

Step 6

Transfer the sheet pan contraption to the oven and bake undisturbed for 45 minutes. Remove the foil (and save it for another use if you’d like). Continue baking until the tomatoes burst, the beans are tender, and the teff is dry around the edges of the pan, about 15 minutes. Serve hot or at room temperature.