Heat the olive oil in the base of a tagine or a heavy-bottomed Dutch oven over medium heat.
Sear the chicken thighs skin-side down until golden brown, about 5 to 7 minutes per side. Remove the chicken from the pot and set aside.
In the same pot, add the sliced onion and sauté until softened, about 5 minutes. Stir in the minced garlic, harissa paste, and star anise, cooking for 1 more minute until highly fragrant.
Pour in the chopped tomatoes and chicken stock. Stir well, making sure to scrape up any browned bits from the bottom of the pot.
Return the chicken thighs to the pot. Cover with the tagine lid and simmer gently on low heat for 40 minutes.
Stir in the chopped rhubarb, cover again, and cook for an additional 15 minutes until the chicken is tender and the rhubarb is softened but still holds its shape.
While the tagine is finishing, prepare the couscous according to the package instructions.
Serve the hot chicken tagine over a bed of fluffy couscous, generously garnished with chopped pistachios and fresh cilantro.