From where do we know that in parashat Ree HaShem is giving us a deal which is either Bracha or Curse, without a middle option?

We know this directly from the pesukim in parashat Re'eh. The pasuk says "Re'eh anokhi noten lifneichem hayom beracha u'klala" - "See, I present before you today a blessing AND a curse." Notice it doesn't say "beracha O klala" - a blessing OR a curse. It says "beracha U'klala" - a blessing AND a curse. This is critical. It's telling us these come together as a package deal. The Torah continues and explains: "Et haberacha asher tishme'u el mitzvot Hashem Eloheichem" - the blessing that you hearken to the commandments of Hashem your God that I command you today. "Vehaklala im lo tishme'u" - and the curse if you do not hearken to the commandments of Hashem your God and you stray from the path to follow gods of others that you did not know. So Hashem is telling us very clearly - there's no middle ground. There's no gray area. You don't get half a bracha or a medium-sized bracha. Either your life is blessed or it's cursed. Simple. If you do what Hashem says, you get the bracha. If you don't, you get the klala. There's no "or nothing" option. This is not free choice where you do whatever you want and the outcomes are neutral. This is choice - you choose good and get good, or you choose bad and get bad. That's the deal.