Step 1

Combine fonio, 1 cup chopped onions, Parmesan, egg, thyme, about 2 teaspoons garlic, 1 1/2 teaspoons salt, and 1/2 teaspoon pepper in a food processor. Process until mixture forms a coarse paste, about 30 seconds. Transfer fonio mixture to a large bowl; add ground beef and 1 cup chopped onions. Using your hands, gently combine meat mixture. Shape into 16 (about 2 1/4-ounces) balls; place on a parchment paper–lined baking sheet. Cover and chill at least 1 hour or up to 8 hours or overnight.

Step 2

Heat 1/4 cup oil in a large high-sided skillet over medium. Add bay leaf, remaining onions (about 2 cups), and remaining garlic (about 2 teaspoons). Cook, stirring often, until onions are golden brown, 15 to 20 minutes. Add tomatoes, 1 cup broth, remaining 3/4 teaspoon salt, and remaining 1/4 teaspoon pepper. Cook, stirring often, until sauce has thickened slightly, 15 to 20 minutes. Stir in sweet potato and remaining 1/2 cup broth. Reduce heat to medium-low; cover and cook, stirring occasionally, until sweet potato is almost tender, 18 to 24 minutes.

Step 3

While sweet potato cooks, heat remaining 2 tablespoons oil in a large skillet over medium-high. Working in batches if needed, cook meatballs in hot oil, turning occasionally, until browned on all sides, about 10 minutes. Transfer to a large plate.

Step 4

Nestle meatballs in skillet with sauce. Cook, uncovered, over medium-low, until sweet potato is tender and meatballs are cooked through, 5 to 10 minutes. Season with salt and pepper to taste. Garnish with Parmesan.