Heat a large pot over medium-high heat. Once hot, add oil, garlic, ginger, and onion. Sauté, stirring occasionally for 5-8 minutes or until the onion has developed a slight sear (browned edges).
Add curry paste and sauté for 1-2 minutes more, stirring frequently. Then add vegetable broth and coconut milk and stir to deglaze the bottom of the pan.
Bring to a simmer over medium heat, then reduce heat to low and cover. Simmer on low for at least 1 hour, (up to 2-3 if you want), stirring occasionally. The longer it cooks, the more the flavour will deepen and develop.
Taste broth and adjust seasonings as needed, adding coconut sugar for a little sweetness, turmeric for more intense curry flavour, or more sesame oil for nuttiness.
About 10 minutes before serving, prepare any desired toppings/sides, such as noodles, sautéed portobello mushrooms, or spring onion (optional). Add any vegetables being used to the broth to cook (you can cook these separately and add on serving if you prefer)
Just before serving, scoop out 1/2 cup of the broth and whisk in the miso paste. Once fully dissolved, add back to the pot and turn off the heat. Stir to combine.
To serve, divide noodles between serving bowls. Top with broth and desired toppings. Serve with chilli garlic sauce or sriracha for added heat.
You can strain the broth when serving if you prefer to not have the onions and ginger in it.
Best when fresh, though the broth can be stored (separate from sides/toppings) in the refrigerator for up to 5 days or in the freezer for up to 1 month.