Wanted to share my version of @jcalz solution that satisfied our ESLint extensive rules and my need of readability, hope it might helps:

/**
 * Create an object type that can only have one key from a KeyUnion type with a specific ValueType
 * @see https://stackoverflow.com/a/60873215
 * @example
 * const correctlyTypedArray : ObjectWithOnlyOneKeyFromUnion<"a" | "b", string> = [{a: "works"}, {b: "works too"}];
 */
export type ObjectWithOnlyOneKeyFromUnion<
    KeyUnion extends string | number,
    ValueType,
    KeyUnionCopy extends string | number = KeyUnion,
> =
    // make an object type with one key present
    {
        [Key in KeyUnion]: { [keyName in Key]: ValueType } & {
            // make all other keys from KeyUnion as optional-and-never
            [keyName in Exclude<KeyUnionCopy, Key>]?: never;
        } extends infer ObjectType
            ? //merge all intersections into a single object type for type linter readability purposes
              { [keyName in keyof ObjectType]: ObjectType[keyName] }
            : never;
    }[KeyUnion];