Bring a large saucepan of salted water to the boil for the ravioli. Thinly slice the onion. Wipe the mushrooms clean, then quarter. Crush or finely chop the garlic. Finely grate the parmesan. Crumble 1 stock cube* into a heatproof jug, add 160ml (⅔ cup) boiling water and stir to dissolve.
Step 2 — Cook onion and mushrooms
Heat 1 tbs olive oil in a medium deep frypan over medium-high heat. Cook the onion and mushrooms, stirring occasionally, for 7-8 mins until caramelised. Reduce the heat to medium.
Step 3 — Start sauce
Add the Tuscan seasoning and garlic to the mushroom mixture. Cook, stirring, for 1 min or until fragrant. Stir in the stock and 1½ tsp balsamic vinegar, scraping to incorporate the flavour from the base of the pan, and bring to a simmer. Cook, stirring occasionally, for 4-5 mins until the sauce is slightly reduced.
Step 4 — Finish sauce
Add 20g butter and 2 tsp milk to the sauce and cook, stirring, for 1 min or until combined. Taste, then season with salt and pepper. Remove the pan from the heat.
Step 5 — Cook ravioli and spinach
While the mushroom sauce is cooking, cook the ravioli in the pan of boiling water for 3 mins or until al dente. Add the spinach to the ravioli, then immediately drain.
Step 6 — Get ready to serve
Add the ravioli and spinach and half the parmesan to the mushroom sauce and gently stir to coat (handle the ravioli with care to ensure they don't break up or burst open). Divide the ravioli and mushroom sauce among bowls. Scatter with the remaining parmesan to serve.