Pound meat to an even thickness, about ¼ inch. Season to taste with salt, pepper, granulated, onion, granulated garlic, and cayenne pepper (ideally several hours before cooking; refrigerate). Dredge in flour. Heat ½ cup oil in a cast-iron skillet over medium-high heat. Working in batches, add meat and cook, turning once, until golden on both sides; remove to a platter. After all the meat is browned, add additional oil if needed to make ½ cup; when hot, whisk in ½ cup of flour and cook, stirring constantly, until a medium brown roux is achieved, 3 to 4 minutes. Add onion, celery, bell pepper, and garlic and cook, stirring, until vegetables are wilted, about 5 minutes. Transfer skillet ingredients to an Instant Pot or other electric pressure cooker set to sauté: high and gradually whisk in the stock, tomatoes, and thyme. Bring to a simmer and cook 5 minutes; taste and adjust seasoning. Add fried meat to the pan, seal, and cook under high pressure for 25 minutes then allow pressure to release naturally for 20 minutes. Carefully remove the lid and check the meat; it should be fork tender. Serve over grits.