Slice the kangaroo fillet into thin slices, three per serving. Brush with olive oil and sprinkle lightly with black pepper.
Remove stalks from spinach and wash leaves thoroughly. Plunge into rapidly boiling water for 30 seconds. Strain and immerse immediately in iced water to stop the cooking process and maintain the green colour. When cold, remove leaves from water and squeeze out as much liquid as possible. Refrigerate until ready to use.
Soften 100 g unsalted butter and blend in food processor with the anchovies, lemon juice and a pinch of sea salt and pepper until smooth. Scrape out onto a piece of foil and form into a sausage shape. Refrigerate until firm.
Heat a large, heavy-base, cast-iron fry pan or grill plate until hot. Toss in the oiled meat slices and quickly sear on each side. Do not turn until the first side is properly sealed (this does not take very long) and don't overcook. Remove meat and rest in a warm place until all the meat slices are cooked.
In another pan, over medium heat, melt the remaining butter, add the squeezed spinach and the salt and pepper, and stir until the spinach is hot. Divide the spinach into four portions, spoon onto the centre of the plate, top with three escalopes. Slice the anchovy butter so it begins to melt over the hot meat. Serve immediately.
Note: Most people won't have ever tasted kangaroo. It is a sweet, strong-tasting meat, it's texture and taste described as somewhere between venison and liver. To eat kangaroo, you have to like game; you have to like offal and you have to be a red meat eater. It's a very big, very strong-tasting meat.