Bring water and brown rice to a boil in a saucepan. Reduce heat to medium-low, cover, and simmer until rice is tender and liquid has been absorbed, 45 to 50 minutes.

When rice is almost finished, heat a large nonstick skillet over medium-high heat. Cook and stir sausage in the hot skillet until browned and crumbly, 5 to 7 minutes. Remove sausage to a large bowl, leaving 1 tablespoon fat in the skillet. Add cooked rice to sausage in the bowl.

Add onion and garlic to the skillet and saute over medium heat until onion is soft and translucent, about 5 minutes. Add celery and cook for 3 minutes. Add collard greens and saute until slightly wilted, 3 to 4 minutes. Add diced tomatoes with their juices, white beans, Cajun seasoning, and red pepper flakes; simmer for 5 minutes. Season with salt and pepper. Serve collard greens over the sausage and rice mixture, or mix it all together.