Bring a pot of lightly salted water to a boil over high heat. Cook the risoni in the boiling water uncovered, stirring occasionally, until tender yet firm to the bite, about 8 minutes. Drain.
Heat 2 tablespoons olive oil in a large frypan over medium high heat; cook and stir the onion and garlic in the hot oil until fragrant, about 1 minute. Crumble the lamb mince into the mixture; season with the oregano, basil and salt and pepper. Continue cooking until the lamb is completely browned, 7 to 10 minutes. Remove from heat. Stir the risoni, spinach, tomatoes, tomato paste and feta cheese into the lamb mixture until evenly incorporated.
Evenly rub the outside of the capsicums and the tops with 2 tablespoons olive oil. Season with salt and pepper. Arrange in a baking dish large enough to accommodate all of them standing upright. Fill the capsicums with the lamb mixture and replace the tops.
Roast in the preheated oven until the capsicums begin to brown, 30 to 40 minutes.