Heat 1 1/2 teaspoons oil in a large nonstick skillet over medium heat. Add half the onion and half the fennel; cook, stirring, until the vegetables are limp and starting to color, about 10 minutes. Pour in half the broth mixture, tuck the spice bag into the vegetables and bring the liquid to a boil. Cook over high heat, stirring occasionally, until the fennel is tender and the sauce has thickened, about 10 minutes. Season with salt and pepper and transfer the mixture to a large bowl, reserving the spice bag.