Chop the onion into pieces slightly smaller than 1/2 inch.
Spray the inside of the slow cooker with nonstick spray or lightly mist with olive oil.
Add the chopped onion, 1 cup brown rice, the 14 oz can of vegetable broth, 1 tsp Greek seasoning, 1/2 tsp Spike seasoning, and 1/2 tsp dried dill weed to the slow cooker and stir to combine.
Cover and cook on high until the rice is nearly tender but still slightly firm, about 1 hour 20 minutes (timing may vary with different rice brands).
While the rice cooks, cut the zucchini and red bell pepper into pieces about 1 inch square so they cook evenly.
When the rice is nearly done, add the diced zucchini and chopped red bell pepper to the slow cooker, cover, and continue cooking on high for another 45–55 minutes, or until the vegetables are tender to your liking.
Meanwhile, crumble about 6 oz feta and reserve a few tablespoons for topping.
When the vegetables are cooked, stir most of the crumbled feta into the rice mixture, reserving a little to sprinkle on each serving.
Serve the slow cooker brown rice hot, topped with the reserved feta.