Cook 3 cups sliced green beans in little water (I add about 1/4 cup) and some salt until tender
Meanwhile, coarsely grind 1/2 cup grated coconut with 2 sliced shallots (or onion), 1/2 tsp cumin seeds, 1/4 tsp turmeric powder and 1/2 tsp red chilli powder without adding any water. Set aside.
Heat 1-2 tsp oil in a pan and add 1/4 tsp black mustard seeds.
When they pop, add the cooked beans
Top up with the grated coconut mixture and a few curry leaves, mix everything together well and adjust salt as needed
Let the thoran cook for 2-3 minutes and remove from heat
That's it! Serve hot with rice and curry
Notes
You can replace red chilli powder with 1-2 green chillies
If you don't have cumin seeds, use half the amount of cumin powder. If using roasted cumin powder, that can alter the flavour so best is to use raw cumin seeds
You can add some sliced onions or shallots to the tempered oil (after mustard seeds), saute till golden brown and then proceed with the rest of the steps
In Tamilnadu poriyal, urad dal is added with mustard seeds while tempering. This is not a thing in Kerala thoran but if you feel the need to, you can add some urad dal
Coriander leaves are not a usual garnish for thoran, but again, if you like it, go ahead and add some
You can make this thoran with a combination of vegetables like cabbage, beans and carrots combined. Make sure to slice or chop them in uniform sizes and if your vegetables have different cooking times, cook the hardiest one first and then add the others later