Add the Panko, grated onion and Worcestershire sauce to a large mixing bowl, then use a fork to mash everything together until the breadcrumbs soak in all the moisture.
Add the remaining ingredients (besides the olive oil) and use your hands to mix everything together. Whilst it's important to ensure everything is evenly distributed and mixed well, try not to squeeze the meat into oblivion, otherwise it can come out tough and dry.
Divide the mixture into 4 equal-sized portions. One by one, roll them into balls, then flatten them into oval-shaped patties (3.5" x 5" is perfect). Use your fingers to smooth over the surfaces and edges, pinching together any little cracks or gaps to help prevent them from splitting as they cook.
Heat a drizzle of oil in a large pan over medium-high heat. Once hot, add the patties and fry for around 3 minutes on each side until nicely browned. The aim is to develop a lovely crust and build flavour, rather than cook them all the way through (they'll finish cooking in the gravy). Remove the patties and place on a plate to one side. You may need to work in two batches depending on the size of your pan.
Remove and discard all but around 1/2 tbsp of fat (just eyeball it), then place the pan back over medium-high heat again. Add the mushrooms and fry until they build up a golden crust, seasoning with a pinch of salt and pepper towards the end. Remove the mushrooms and place to one side, then lower the heat to medium.
Melt the butter, then fry the garlic for 30 seconds or so (be careful it doesn't burn). Stir in the tomato paste and fry for a minute or so, then stir in the flour. Gradually add the stock, whisking as you go to ensure no lumps form. Add 1 tsp Worcestershire sauce, then stir in the mushrooms and any resting juices from the patties.
Nestle in the patties, then gently simmer for 8-10 minutes until the gravy thickens and the patties are cooked through. It will thicken, just keep gently simmering away, spooning over the patties as you go.
Serve up and enjoy!