Our custom today among Ashkenazim is to not recite a Bracha before Hallel at the Seder. One popular explanation for the practice is that taking a break in the middle of reciting Hallel to enjoy the Seder meal precludes reciting a Bracha before Hallel. However, among our Rishonim, Medieval rabbinic authorities, there was a disagreement about the practice. The following excerpt from a French Ashkenaz Haggadah summarizes several of the positions:
Translation: There are those who recite the Bracha Ligmor Et Ha'Hallel at this point but go out and see which practice the majority accepted. Others recite the Bracha of Likro and then after the meal they begin the second half of Hallel with the Bracha of Ligmor. See the Tashbetz (customs of the Maharam of Rottenberg). Our custom is to recite the Bracha of Ligmor at this point (before the first half of Hallel) and after the meal, we begin the second half of Hallel without a Bracha.
The Tur presents the various customs and then suggests a solution to the issue:
טור אורח חיים הלכות פסח סימן תעג'- בענין ברכת ההלל איכא פלוגתא דרבוותא ריצב"א היה מברך עליו ב"פ אחת קודם אכילה ואחת אחר אכילה וכן היה נוהג ה"ר מאיר מרוטנבורק וכ"כ רב האי ורב צמח ורב עמרם אבל הרי"ץ גיאת ואבי העזרי כתבו שאין לברך עליו כלל לפי שחולקים אותו לשנים לפני הסעודה ולאחריה וא"כ האיך יברכו כיון שפוסקים באמצע וכן היה נוהג א"א הרא"ש ז"ל וכן ראוי לעשות בכל דבר שיש ספק בברכתו שאין לברך דברכות אינן מעכבות ויש מקומות שנוהגין לקרות ההלל בב"ה בציבור כדי שלא יצטרכו לברך עליו בשעת ההגדה ומה טוב ומה נעים ההיא מנהגא ויש לו סמך במסכת סופרים דאיתא התם תניא ר"ש בן יוצדק אומר י"ח ימים ולילה אחת גומרין בהן ההלל ובגולה כ"א יום וב' לילות ומצוה מן המובחר לקרות ההלל בב' לילות של גליות ולברך עליו ולאמרה בנעימה ולקיים מה שנ' ונרוממה שמו יחדיו וכשהוא קורא בביתו א"צ לברך שכבר בירך עליו.
Translation: Concerning the question as to reciting a Bracha before Hallel at the Seder, there is a dispute among Rabbinic authorities. Ritzba held that a Bracha should be made before each half of Hallel. That practice was followed by the Mharam M’Rottenberg and so too wrote Rav Hai Gaon, and the Gaonim Rav Tzemach and Rav Amrom. But the Ritz Gayit and my father wrote that no Bracha should be recited over either half of Hallel since we spit the recital in two. Their objection is expressed as how can we make a Bracha when we take a break in the middle of reciting all the chapters of Hallel. And that is how my father, the Rosh, z”l conducted himself. I agree with his position because that is how we act when we are in doubt as to whether we need to recite a Bracha since omitting a Bracha does not invalidate the performance of the Mitzvah.
Certain places follow the practice of reciting Hallel communally in synagogue so that they avoid the whole question as to whether a Bracha is necessary when reciting either half of Hallel at the Seder. How wonderful a custom that is. Support for the practice can be found in Masechet Sofrim where we find that R. Shimon son of Yutzdak said there are 18 days and one night in which Hallel is recited in Eretz Yisreol and 21 days and one night in the Diaspora. It is a wonderful way to perform the Mitzvah when it is fulfilled communally on the two first nights and to recite a Bracha in advance of doing so and then reciting Hallel pleasantly and to thereby glorify G-d as a group. If a person follows that practice, he avoids the necessity of reciting a Bracha before Hallel at the Seder since he already fulfilled the requirement to recite Hallel on the first two nights of Pesach.
I am attaching two images from the Darmstadt Haggadah (1430, Germany) in which the custom to recite a Bracha before both halves of Hallel was followed:
