Flex grid, horizontal with a vertical stack?

Question

JSFiddle

I need to have a flex grid that looks like:

1|2
1|3

Item 1 is the entire height and items 2 and 3 are half the height. Is this possible without creating 2 flex grids?

ul {
    display: flex;
    flex-wrap: wrap;
    list-style-type: none;
    margin: 0;
}
li {
    flex-basis: 25%
}
li:first-child {
    flex-basis: 50%;
}

<ul>
    <li>1</li>
    <li>2</li>
    <li>3</li>
</ul>