Heat the olive oil in a very large pot. Sear marrow bones for a few minutes, turning until well browned.

Peel and roughly chop the carrots, parsnips, parsley roots, and knob celery. Add the chopped vegetables and garlic to the pot and sauté for five minutes, until fragrant.

Add the squash, onions, tomato, chicken bones, peppercorns, salt, and water. Bring to a boil, then lower heat and simmer for eight hours.

Allow the liquid to cool slightly. Strain and discard the solids. Transfer to containers and refrigerate or freeze until ready to use.

Preheat oven to 350 degrees Fahrenheit (175 degrees Celsius).

Spread the onion along the bottom of a baking dish. Arrange club steaks on top. Drizzle with olive oil and season with salt and pepper.

Scatter carrots, parsley roots, shallots, and garlic around the meat. Pour 1/2 cup bone stock into the bottom of the pan. Cover tightly and bake for one to one and 1/2 hours, until the vegetables are fork-tender.

Remove from the oven and transfer club steaks to a plate. Place all vegetables, along with the pan drippings, into a food processor and blend until completely smooth.

Place vegetable puree into a saucepan. Add two to three cups bone stock and red wine and bring to a boil.

Mix potato starch with water and stir into the gravy. Cook until thick and smooth, adding more stock if the gravy is too thick or additional slurry if needed. Adjust salt to taste.

Lower oven temperature to 225 degrees Fahrenheit (105 degrees Celsius).

Return meat to the baking dish, pour the gravy over it, cover, and bake for one to two hours, or until meat reaches desired

softness. Serve over mashed potatoes.