Prep the hearts, rinse the hearts, then cut in half lengthwise, remove the stringy portions, fat, and trim the top off with the tubes.
Slice each prepped heart into 1 inch cubes. This is two hearts cubed, 3 cups and just about 600 grams (just over 1 lb).
Add the cubes to a ziplock bag, add the salt and pepper as desired, olive oil, and seasonings of choice, parsley, thyme, and rosemary work very well with this. Zip the bag closed and mix the pieces well with the marinade. Place in the fridge overnight. (I used a plastic dish to hold the cubes as I cut the hearts, then mixed the olive oil and seasonings in this, then poured them into a zip lock bag.)
When you are ready to cook, heat a non stick pan on medium high heat.
Place a strainer over a bowl and empty the contents of the zip lock bag into the strainer, reserve the marinade and set that aside for now.
Empty the strainer now into the hot pan, brown the pieces on all sides. (The heart cubes are covered in oil to start with and they release some water as well, no additional liquid is added to the pan.)
Add the leftover marinade, if any, to the pan and stir the heart pieces into the liquid. Reduce the heat to medium low, cover pan, and cook, stirring occasionally, for about 25 minutes.
Add the carrot and onion to the pan and stir them in. Cover and simmer for about 30 minutes or until the carrots are tender.
Carrots are tender, ready to serve.
Served as is but keep in mind, you can add this to a stew, soup, mix in some chicken gravy, etc.