Wash, peel and cook potatoes in 2 litres of water over a medium high heat until the potatoes are fork tender. Drain and allow to cool enough to handle. Cut into quarters and set aside.
While the potatoes are cooking, prepare the spice paste by blending the onion, half of the chopped tomatoes, and sambal oelek or red jalapeños in a food processor until very smooth, adding a little water if needed to keep the blades moving. Set aside.
Heat the oil in a frying pan over a medium heat. Add the ground spice paste and allow it to cook, stirring frequently, for 10 minutes, until the oil separates and rises to the surface. Add the tender stem broccoli, coconut cream, water, fish sauce, and sugar. Stir, and cook for another 10 minutes, until the oil separates and rises to the surface again (indicating that the spice mixture – sambal - is cooked). Taste and add more sugar for sweetness or fish sauce for saltiness as desired.
Add the potatoes to the frying pan and stir gently to coat with the sambal. Place the cooked potatoes in an ovenproof dish and top evenly with the cooked spice paste. Bake for approximately 25 minutes. Turn off the heat. Carefully remove the casserole from the oven and serve warm. Garnish with the remaining chopped tomatoes and coriander.