If your bell peppers do not sit upright on the cutting board, trim about 1/8 inch from the bottom to create a level surface. Remove the top 1/2 inch of each pepper, discard the stems, and finely chop the remaining tops. Remove the seeds and inner membranes, setting the hollowed peppers aside.

Using a 6-quart Instant Pot, select the high sauté setting. Add olive oil to the pot and, once it shimmers, sauté the chopped bell pepper tops, onion, garlic, 1/2 teaspoon salt, and several grinds of black pepper. Stir occasionally and cook for about 4 minutes until the vegetables are softened. Add the ground beef and continue cooking, breaking it into small pieces with a spoon until it is no longer pink, about 4 minutes.

Stir in the diced tomatoes, chicken broth, tomato paste, chili powder, thyme, 1 teaspoon salt, and a few more grinds of pepper. Continue to cook, stirring occasionally, until the tomatoes soften and the mixture thickens to a saucy consistency, approximately 3-4 minutes. Mix in 1 cup of the shredded Cheddar and the pre-cooked rice, stirring until the cheese melts and everything is well combined. Adjust seasoning if needed. Turn off the sauté function.

Fill the reserved bell peppers with the beef mixture. Clean the Instant Pot bowl and place the rack at the bottom. Pour in 3/4 cup of water. Arrange the stuffed peppers snugly in the pot.

Follow your Instant Pot's instructions to lock the lid and set it to pressure cook on high for 5 minutes. Once the cooking cycle finishes, use the quick-release method as outlined in the manufacturer's guide (this will take approximately 1 minute). After releasing the pressure, carefully remove the lid, top each pepper with the remaining 1/2 cup of shredded Cheddar, and place the lid back on. Allow the residual steam to melt the cheese for about 1 minute. Use tongs or a large spoon to carefully transfer the stuffed peppers to a plate and garnish with freshly chopped parsley.