Heat the broth in a tagine or in a thick-bottomed pot with a lid.
Add the spices/herbs and seasonings to the broth and whisk to ensure there are no lumps. You can use a real whisk if you want, but I just use a fork.
Finely dice the onion and place into the seasoned broth. Simmer, covered.
Finely dice the chillis and add to the simmering onions. Keep the seeds if you like your food hot, otherwise remove the seeds before dicing.
While this is simmering, dice the zucchini and yellow squash. Then cut the bell peppers into long strips. Set both aside in a bowl.
Cut chicken breast in half the long way. Take each half and slice along the short axis so that the slices are not more than 1/4" thick. This is easiest to do if the chicken is partially frozen.
Add the chicken to the simmering onions and chillis, ensuring that the slices are submerged.
Add the zucchini, yellow squash, and bell peppers. Do not stir. It's ok if they are not submerged, the steam will cook them.
Re-cover and simmer 45 minutes. Serve over couscous or rice. Alternatively, remove chicken and veggies, increase heat to "high", and reduce the sauce. Add chicken and veggies back, heat throughout, and serve as a stew.