This Southeast Asian-inspired bowl is all about big flavor with minimal effort. When working with lemongrass, give the stalk a couple of whacks with the back of a knife before chopping to release its flavorful oils.
Place 3/4 cup coconut milk, lemongrass, sambal oelek, and chicken in a large zip-top plastic bag. Seal and turn to coat; refrigerate 8 to 24 hours.
Place stock, garlic, onions, carrots, and ginger in a 6-quart slow cooker. Remove chicken from marinade; discard marinade. Arrange chicken on top of onion mixture. Cover and cook on LOW 7 1/2 hours. Remove chicken from slow cooker; shred chicken into bite-size pieces. Discard bones.
Pour cooking liquid through a colander arranged over a large bowl; discard solids. Combine strained cooking liquid, soy sauce, fish sauce, lime juice, sugar, and remaining 1/2 cup coconut milk in a small saucepan over medium-high heat. Bring to a boil, stirring until sugar dissolves. Remove pan from heat.
Prepare noodles according to package directions. Divide noodles evenly among 6 bowls. Top evenly with chicken, cucumber slices, cilantro, and green onions. Pour 1/3 cup coconut milk mixture over each serving; sprinkle servings evenly with peanuts.