@Serializable
data class Teaser(
    val text: Text
) {
    @Serializable
    data class Text(val content: String)
}

@Serializable
@JvmInline
value class Text(val content: String)

Json.decodeFromString<Teaser>("""
{
  "text": "blah blah"
}
""")

{
  "text": {
    "content": "g"
  }
}

Teaser(text=g)

@Serializable
data class Teaser(
    @SerialName("text")
    val textObj: Text
) {
    constructor(text: String) : this(Text(text))

    @Transient // not sure if this is needed
    val text: String
        get() = textObj.content

    @Serializable
    data class Text(val content: String)
}

Serializable