The marinade is the secret. The chicken (or fish) is soaked overnight in lemon juice, mustard, onions, garlic, oil and a little chilli. By the next morning, it has soaked up so much flavour that even before it is cooked, the kitchen smells amazing.
Then comes the onion mountain. Lots of onions - more than you'd think - are sliced thinly and cooked slowly until they go soft and sweet. The chicken is added back in, the sauce thickens, and it is poured over plain rice in a big shape that looks a bit like a snow-capped hill.
Yassa is sometimes called 'the national dish of welcome'. If a guest is coming to your house in Senegal, yassa is often what gets cooked. The slow cooking, the smell of lemon and onion - it tells the guest: 'we spent time on this, just for you.'
Different regions tweak the recipe. In coastal areas, fishermen often make yassa with the day's catch. Inland, chicken is more common. Some families add olives. Some add a little extra mustard. As with most beloved dishes, every grandmother has the 'real' version.