Grilled Shrimp and Summer Squash

2 tsp. Dijon mustard  

Grated zest of 1 lemon 

2 Tbs. fresh lemon juice  

1/3 cup extra-virgin olive oil, plus more for brushing 

1 small serrano chili, seeded and minced 

1 tsp. minced fresh marjoram 

1 Tbs. minced fresh basil 

Coarse kosher salt and freshly ground pepper, to taste 

1 1/4 lb. colossal shrimp, peeled and deveined with tails intact 

8 summer squash, cut lengthwise into 3 pieces each