While vegetables are roasted, make the lemon mustard cream sauce. In a small saucepan over medium-low heat, melt 2 tablespoons butter. Reduce heat to low, and whisk in flour (called a roux). Whisk constantly to prevent flour from burning or sticking, until well incorporated and slightly browning, about 10 minutes.
Pour in milk slowly and season with salt and pepper. Increase heat to medium and keep whisking to incorporate flour mixture until fully incorporated and sauce begins to thicken, about 10 minutes. Remove from heat and whisk in mustard and lemon juice.