Pour broth into a pot large enough to hold the shrimp and bring to a boil over high heat. Add shrimp and poach at a simmer, lowering heat if needed, until firm and pink, about 5 minutes.
Remove the shrimp with a slotted spoon and set them aside on a plate.
Keep the broth simmering over low heat.
Heat olive oil and melt 2 tablespoons of the butter in a large, heavy bottomed pot over medium high heat until hot, but not smoking.
Add onion and cook for about 4 minutes.
Add wine, bring to a boil and cook, stirring, until nearly all of the wine has evaporated.
Add rice and ladle 1 cup of summer broth into rice and cook, stirring, until it is nearly absorbed.
Continue to add broth in 1/4 cup increments, stirring rice constantly.
When you are down to the last 2 cups or so, add the broth in smaller increments until the rice is softened but still a bit al dente. (add or delete broth as needed). This step should take about 18 minutes.
Stir in 1 Tblsp. lemon zest and 1 Tbslp. lemon juice. Taste and adjust with additional zest or juice as needed until the risotto attains a nice, bright flavor.
Stir in the reserved shrimp, tarragon and remaining 2 Tbslp butter.