Roasted Mahimahi with Citrus Vinaigrette

1 lemon 

Olive oil for brushing 

4 mahimahi fillets, each 5 to 6 oz.  

4 tsp. grated lemon zest  

4 tsp. grated lime zest  

Coarse kosher salt and freshly ground pepper, to taste 

2 tsp. Dijon mustard  

1/3 cup extra-virgin olive oil  

1 shallot, minced