Despite the fearsome smell when it’s collected together in bulk, modern garum in the form of Colatura di Alici is a genuinely magical ingredient. It’s very similar to what the Romans used, except it’s made exclusively from anchovies whereas the Romans used many varieties of fish to make garum. I use it often in pasta dishes especially, but it’s a versatile ingredient for adding umami in general. It’s similar to fermented fish sauces found in South East Asia, while being quite distinct from them.
Max Miller does a lot with garum, he even made some at one point (not a process for the faint hearted!) and it was his and Historical Italian Cooking’s dishes where I first learned about colatura. Highly recommend them both, another ingredient I’ve become fond of through Roman cooking is long pepper. Not too sure about its global availability but in the UK I buy it from Indian shops. Parthian Chicken is another excellent dish, although some have an acute sensitivity to the smell of asafoetida resin as my unfortunate housemate discovered! Asafoetida is actually a substitute for silphium, a herb harvested to extinction by the Romans themselves.