Pound the chillies and garlic together in a mortar and pestle. You want a coarse paste. Finely chop if you don't have a mortar and pestle. Set aside.
Whisk together all the ingredients that go into making the sauce. Set aside.
Add oil to a large wide pan. Sauté the coarsely pounded chilli-garlic paste until aromatic, about 30 seconds. Add the beef and sugar to the pan and cook until browned. (The beef will first release a lot of its moisture before it starts to brown.)
Add the onions and peppers to the beef and cook until they soften, about 7 mins. Pour in the sauce and toss to coat. Cook until the sauce thickens enough to just coat the beef in a sticky, glossy glaze. Taste and adjust seasoning.
Stir in the basil leaves and toss until just wilted, about 30 seconds.
Serve with steamed rice, a runny egg, and steamed green beans/broccoli alongside.
Notes
Use Holy basil if you can’t find Thai basil. Italian basil will give you a different flavour but it’ll still work too.