Restaurant is a neuter noun. When possessive pronouns precede a noun they decline like articles. So unseres could be used for a neuter noun (and a masculine noun) but only in the genitive case. In this case because it is nominative neuter it remains unser. "Eigen in the sentence is an adjective and thus declines differently. Based on the mixed declension rules (because it follows the possessive pronoun unser) it becomes eigenes.
Take a look at this website for the possessive pronoun declensions:
and this one for the adjective declension tables: