In this answer, I denote the cartesian unit vectors by $\mathbf{e}_i$ ($i=1,2,3$) and the radius vector $\mathbf{r}=\sum_ix_i\mathbf{e}_i$.
Assuming $\mathbf{L}=\frac{1}{\mathrm{i}}\mathbf{r}\times\nabla$ we have \begin{align}
\nabla\times\mathbf{L}&=\sum_{i,j,k}\varepsilon_{ijk}\mathbf{e}_i\partial_jL_k\\
&=\sum_{i,j,k}\varepsilon_{ijk}\mathbf{e}_i\partial_j\left(\frac{1}{\mathrm{i}}\mathbf{r}\times\nabla\right)_k\\
&=\frac{1}{\mathrm{i}}\sum_{i,j,k}\varepsilon_{ijk}\mathbf{e}_i\partial_j\sum_{l,m}\varepsilon_{klm}x_l\partial_m\\
&=\frac{1}{\mathrm{i}}\sum_{i,j,k,l,m}\varepsilon_{kij}\varepsilon_{klm}\mathbf{e}_i\partial_j\left(x_l\partial_m\right).
\end{align} Where I used $\varepsilon_{ijk}=\varepsilon_{kij}$. Now \begin{equation}
\sum_k\varepsilon_{kij}\varepsilon_{klm}=\delta_{il}\delta_{jm}-\delta_{im}\delta_{jl}
\end{equation} so \begin{align}
\nabla\times\mathbf{L}&=\frac{1}{\mathrm{i}}\sum_{i,j,l,m}\left(\delta_{il}\delta_{jm}-\delta_{im}\delta_{jl}\right)\mathbf{e}_i\partial_j\left(x_l\partial_m\right)\\
&=\frac{1}{\mathrm{i}}\sum_{i,j,l,m}\delta_{il}\delta_{jm}\mathbf{e}_i\partial_j\left(x_l\partial_m\right)-\frac{1}{\mathrm{i}}\sum_{i,j,l,m}\delta_{im}\delta_{jl}\mathbf{e}_i\partial_j\left(x_l\partial_m\right)\\
&=\frac{1}{\mathrm{i}}\sum_{i,j}\mathbf{e}_i\partial_j\left(x_i\partial_j\right)-\frac{1}{\mathrm{i}}\sum_{i,j}\mathbf{e}_i\partial_j\left(x_j\partial_i\right).
\end{align} To rewrite the summands, let's evaluate their action on a twice continuously differentiable scalar field $f(x_1,x_2,x_3)$. We get \begin{align}
\partial_j\left(x_j\partial_if\right)&=(\partial_jx_j)(\partial_if)+x_j\partial^2_{ij}f\\
&=\delta_{jj}\partial_if+x_j\partial^2_{ij}f.
\end{align} Similarly \begin{align}
\partial_j\left(x_i\partial_jf\right)&=(\partial_jx_i)(\partial_jf)+x_i\partial^2_{j}f\\
&=\delta_{ij}\partial_jf+x_i\partial^2_{j}f.
\end{align} Since $f$ was arbitrary enough we have the operator equalities \begin{equation}
\partial_j\left(x_i\partial_j\right)=\delta_{ij}\partial_j+x_i\partial^2_{j},\quad \partial_j\left(x_j\partial_i\right)=\delta_{jj}\partial_i+x_j\partial^2_{ij}.
\end{equation} Hence \begin{align}
\nabla\times\mathbf{L}&=\frac{1}{\mathrm{i}}\sum_{i,j}\mathbf{e}_i\left(\delta_{ij}\partial_j+x_i\partial^2_{j}\right)-\frac{1}{\mathrm{i}}\sum_{i,j}\mathbf{e}_i\left(\delta_{jj}\partial_i+x_j\partial^2_{ij}\right)\\
&=\frac{1}{\mathrm{i}}\sum_{i,j}\mathbf{e}_i\delta_{ij}\partial_j+\frac{1}{\mathrm{i}}\sum_{i,j}\mathbf{e}_ix_i\partial^2_{j}-\frac{1}{\mathrm{i}}\sum_{i,j}\mathbf{e}_i\delta_{jj}\partial_i-\frac{1}{\mathrm{i}}\sum_{i,j}\mathbf{e}_ix_j\partial^2_{ij}\\
&=\frac{1}{\mathrm{i}}\nabla+\frac{1}{\mathrm{i}}\mathbf{r}\nabla^2-\frac{3}{\mathrm{i}}\nabla-\frac{1}{\mathrm{i}}\sum_{i,j}\mathbf{e}_ix_j\partial^2_{ij}\\
&=-\frac{2}{\mathrm{i}}\nabla+\frac{1}{\mathrm{i}}\mathbf{r}\nabla^2-\frac{1}{\mathrm{i}}\sum_{i,j}\mathbf{e}_i\partial_i(x_j\partial_j)+\frac{1}{\mathrm{i}}\sum_{i,j}\mathbf{e}_i\underbrace{\partial_i(x_j)}_{=\delta_{ij}}\partial_j\\
&=-\frac{2}{\mathrm{i}}\nabla+\frac{1}{\mathrm{i}}\mathbf{r}\nabla^2-\frac{1}{\mathrm{i}}\sum_{i,j}\mathbf{e}_i\partial_i(x_j\partial_j)+\frac{1}{\mathrm{i}}\sum_{i}\mathbf{e}_i\partial_i\\
&=-\frac{2}{\mathrm{i}}\nabla+\frac{1}{\mathrm{i}}\mathbf{r}\nabla^2-\frac{1}{\mathrm{i}}\nabla(\mathbf{r}\cdot\nabla)+\frac{1}{\mathrm{i}}\nabla\\
&=\frac{1}{\mathrm{i}}\mathbf{r}\nabla^2-\frac{1}{\mathrm{i}}\nabla-\frac{1}{\mathrm{i}}\nabla(r\partial_r)\\
&=\frac{1}{\mathrm{i}}\left[\mathbf{r}\nabla^2-\nabla(1+r\partial_r)\right],\quad\text{Q.E.D.}
\end{align}