Green Beans with Walnuts and Mustard Dressing

The Food 52 Cookbook Club cookbook for December is Alison Roman's new cookbook, Nothing Fancy.  I usually do not work ahead, but I needed a green bean recipe for 50 dinner guests that could be served room temperature and made ahead.  This fit the bill, and the recipe scales up well.

1 cup raw walnuts, chopped
A squeeze of Nam Pla (this can be omitted)
2 cloves garlic, minced
 1.5 lb. green beans
1 lemon, thinly sliced
salt and pepper
2 Tbs whole grain mustard
2 Tbs. white wine vinegar
1/2 cup fresh dill finely chopped (other herbs can be substituted)

Preheat oven to 450 degrees.  Toss the  green beans and lemons with enough olive oil to coat, with some salt and pepper.  Put on a 1/2 sheet pan in a single layer and roast about 15 minutes.
Heat the walnuts in a 1/4 cup (or less) olive oil with garlic, and stir until walnuts are toasted.  Squirt in a little nam pla, toss until evaporated and distributed, and turn off heat.
Combine mustard, vinegar, 1/4 c. olive oil, salt and pepper.  Mix then toss with the green beans.  Put them on a platter, scatter walnuts and herbs over the top.  Serve.