Prep your ingredients. Cut chicken into even pieces, chop vegetables, and measure out spices. Having everything ready will make the cooking process much smoother.
Heat oil in a large pot over medium-high heat until shimmering but not smoking. Add chicken pieces and cook until lightly browned on all sides, about 5-6 minutes. Remove chicken and set aside.
Reduce heat to medium and add curry paste to the same pot. Stir constantly for 1-2 minutes until fragrant and slightly darkened, being careful not to burn it.
Add onions, garlic, and ginger to the pot and sauté for 2-3 minutes until onions begin to soften and become translucent.
Return chicken to the pot along with pumpkin cubes and bell pepper. Stir to coat everything with the curry paste mixture.
Pour in coconut milk and stir to combine. Bring the mixture to a gentle simmer, then reduce heat to medium-low.
Add fish sauce and brown sugar, stirring to incorporate. Allow curry to simmer uncovered for 20-25 minutes or until pumpkin is tender and chicken is fully cooked.
Taste and adjust seasonings as needed – add more curry paste for heat, fish sauce for saltiness, or brown sugar for sweetness.
Remove from heat and stir in lime juice. The acidity balances the richness of the coconut milk and brightens all the flavors.
Serve hot over jasmine rice, garnished with fresh Thai basil or cilantro leaves and lime wedges on the side.