Place tofu, in a single layer, in a ceramic dish. Combine 2 tbs kecap manis and 1 tbs sweet chilli sauce and spoon over tofu. Turn to coat. Cover and refrigerate 30 minutes if time permits. Preheat a large non-stick frying pan on medium-high. Brush pan with a little oil and cook tofu, in two batches, for 3-4 minutes each side or until caramelised. Transfer to a plate, keep warm.
Heat a wok over medium-high heat until hot. Add 3 tsp oil, onions, garlic and ginger and stir-fry 30 seconds until aromatic. Add remaining oil and mushrooms, stir-fry 3-5 minutes until almost tender. Stir in stock and bring to the boil. Reduce heat to medium-low and simmer 5 minutes.
Stir in remaining kecap manis, chilli sauce and noodles and simmer further 5 minutes until noodles warmed through. Add spinach and cook 1 minute until wilted.
Cut tofu slices and put into base of serving bowls. Ladle over soup and serve.