Cook the Aromatics: In a large pot, heat oil over medium heat. Add the ginger paste and minced garlic, sautéing for 1-2 minutes until fragrant.
Add Curry Paste: Stir in the red curry paste and cook for another 1-2 minutes to let the flavors develop.
Make the Broth: Pour in the coconut milk, fish sauce, and 1 1/2 to 2 cups of chicken or vegetable broth (starting with 1 1/2 cups, adding more if you prefer a thinner soup). Add the lime juice, brown sugar, red pepper flakes, and salt to taste. Stir to combine and bring to a simmer.
Poach the Chicken: Add the sliced chicken thighs directly to the simmering broth. Allow the chicken to cook for 5-7 minutes, or until cooked through and tender.
Add Vegetables: Stir in the red bell pepper and green beans. Simmer for an additional 5-7 minutes, until the vegetables are tender but still have a slight crunch.
Finish with Herbs and Noodles: Add the Thai basil, cilantro, and cooked udon noodles. Stir and cook for another 1-2 minutes, until the noodles are warmed through.
Serve: Taste the soup and adjust seasoning as needed. Ladle into bowls and garnish with additional herbs if desired. Enjoy hot!