Vegetarian dishes that feel thin rarely lack seasoning or spice. What they lack is savoury depth, and the compounds responsible for it are less abundant in plants than in meat.
Umami comes from specific free compounds
The savoury taste is triggered mainly by free glutamate, an amino acid that has been released from the protein chain it was originally part of.
Certain nucleotides produce the same sensation, and when both are present together the effect is far stronger than either alone rather than simply additive.
Meat, aged cheese and fish carry both in useful quantities, which is why a small amount of any of them can lift an entire dish.
Plants hold their glutamate bound up
Vegetables and pulses contain plenty of protein and therefore plenty of glutamate, but most of it is locked inside intact protein chains where the tongue cannot detect it.
Something has to break those chains before the taste appears, and in the absence of ageing or fermentation the cooking has to do that work in a short time.
This is the actual reason a quickly cooked vegetable dish can taste flat despite good ingredients and correct seasoning.
Fermentation is the traditional solution
Microbial and enzymatic action breaks proteins down over days or months, releasing free glutamate and producing the depth that fermented foods are valued for.
Every major vegetarian cooking tradition includes fermented staples of this kind, and they are used in small quantities precisely because they are concentrated.
Sour batters, aged pickles, fermented pastes and yoghurt all contribute this effect alongside their acidity, which is part of why they are so difficult to substitute out.
Browning and reduction build it in the pan
Deep browning of onions, tomatoes and pastes generates savoury compounds through heat rather than time, which is why the long fried base of so many dishes is not optional.
Reduction concentrates whatever free compounds are present by removing water, so a sauce cooked down tastes savoury in a way the same sauce diluted does not.
Both take time on the stove, and this is where hurried vegetarian cooking most often loses its depth.
Certain ingredients carry it naturally
Dried mushrooms, tomatoes, seaweed, aged cheeses and toasted lentils are unusually high in the relevant compounds and act as concentrated additions.
Combining two sources tends to beat doubling one, because the glutamate and nucleotide pathways amplify each other rather than simply stacking.
A dish built with a fermented element, a browned base and one naturally rich ingredient rarely needs anything else to feel complete.