1. Prepare the rice by combining rice, water, 1 tablespoon butter, and salt in a pot. Bring to a boil, reduce heat, cover, and cook for 15 minutes. Turn off heat and let the rice rest for 10 minutes.
2. For the harissa broth, heat 2 tablespoons of oil in a wide pot over medium heat. Add shallots and cook until softened, then add garlic and potatoes, cooking for a few more minutes.
3. Stir in harissa, cayenne, and paprika, cooking for another minute to release flavors. Pour in vegetable stock and bring to a boil.
4. Reduce to a simmer and cook for about 20 minutes until potatoes are tender. Add cream or coconut milk, and season with lemon juice, salt, and pepper. Finally, add green beans and cook until tender-crisp.
5. If using roasted potatoes, preheat the oven to 400°F, toss potatoes with olive oil, paprika, salt, and pepper, and roast on a baking sheet cut side down until golden brown.
6. To serve, place rice in bowls and ladle harissa broth with vegetables over it. Garnish with chopped cilantro if desired.