Peel the onion and carrot. Roughly chop the onion, carrot, and celery stalk into large chunks.

Heat the oil in a medium pot over medium-high heat. Add the beef and chicken in a single layer and sear for 3–4 minutes per side until well browned.

Add the chopped onion, carrot, and celery to the pot. Cook for 3–4 minutes, stirring occasionally, until the vegetables soften slightly and pick up some color.

Pour in a small splash of the cold water and scrape the bottom of the pot with a wooden spoon to release any browned bits.

Add the remaining cold water, bay leaf, black peppercorns, salt, garlic clove, and thyme or parsley if using. Ensure the meat and vegetables are just covered with water.

Bring to a gentle simmer over medium-high heat, then reduce the heat to low to maintain a very gentle simmer. Skim off any foam that rises to the top.

Simmer uncovered for 60–75 minutes, skimming occasionally, until the stock tastes rich and meaty and has reduced slightly. If you have a slow cooker you can do it on low for 2 hours.

Remove the meat and vegetables with tongs, then strain the liquid through a fine-mesh sieve into a clean pot or heatproof bowl. Discard aromatics. Reserve the meat if desired for another use.

Taste the stock and adjust seasoning with more salt if needed. Let cool slightly, then refrigerate until cold. Skim off any solidified fat from the surface if you prefer a leaner stock.

Use immediately or store in the fridge for up to 3 days, or freeze in small containers or ice cube trays for up to 3 months.