Warm the vegetable stock in a small saucepan over low heat and keep it at a bare simmer near the stove.
Melt the olive oil and 40 g butter in a 3-quart / 2.8 L heavy-bottomed pot over medium heat. Add the onion and cook 3 to 4 minutes until soft and translucent.
Add the shredded cabbage with a pinch of salt. Cook 6 to 7 minutes, stirring often, until wilted and reduced by roughly half.
Add the rice and stir to coat every grain in fat. Toast for 2 minutes until the edges look glassy.
Pour in the wine and stir until it's absorbed, about 1 to 2 minutes, scraping up any browned bits from the bottom.
Begin adding the warm stock one ladle (about 120 ml) at a time, stirring frequently and waiting until it's mostly absorbed before adding the next. This takes 16 to 18 minutes total.
Start tasting the rice around 16 minutes. It's done when the center has a slight bite and the mixture looks loose and pourable, not stiff.
Take the pot off the heat. Stir in the remaining 20 g butter and the grated Parmesan until melted and glossy. Season with black pepper and more salt if needed.
Let it rest 2 minutes off heat, then spoon into shallow bowls right away and top with extra Parmesan.