Since when is sushi a substitute for a steak?

The Torah tells us that Bn"Y complained מִ֥י יַאֲכִלֵ֖נוּ בָּשָֽׂר, that they wanted meat, not just mon to eat.  Then the people added, זָכַ֙רְנוּ֙ אֶת־הַדָּגָ֔ה אֲשֶׁר־נֹאכַ֥ל בְּמִצְרַ֖יִם חִנָּ֑ם אֵ֣ת הַקִּשֻּׁאִ֗ים וְאֵת֙ הָֽאֲבַטִּחִ֔ים וְאֶת־הֶחָצִ֥יר וְאֶת־הַבְּצָלִ֖ים וְאֶת־הַשּׁוּמִֽים׃ (11:5), that they remembered the free fish and vegetables that they ate while in Egypt.  What's the connection between the desire for meat and remembering the fish and vegetables of Egypt?  A sushi roll and vegetable platter are no more a substitute for meat than the mon was, so why is it relevant?

Secondly, the mon tasted like any food in the world that you could imagine.  If you wanted a steak, you could think of steak when eating the mon and it would taste like steak.  Why then did the people complain?

Alshich writes that the mon could taste like steak, but it wasn't steak.  It did not have the same nutrients and vitamins.  You can dress up and impossible burger to look and taste just like a hamburger, but you probably need to take iron supplements and other vitamins if that's your whole diet.  

Bn"Y were feeling run down -- nafsheinu y'veisha -- and felt it must be due to the diet of mon lacking something.  Kal v'chomer, they said.  We remember eating the poor diet of Egypt, the fish that was so bad they were giving it away for free, and we still had the stamina to do the work of slaves, so why now are we feeling so run down?  It must be that we need real food, not just mon.

Chazal comment that the complaint for meat was not about food per se, but was about the obligations that now came with the food.  They comment on  אֲשֶׁר־נֹאכַ֥ל בְּמִצְרַ֖יִם חִנָּ֑ם that the word חִנָּ֑ם, free, here does not mean it was without cost.  It does not make sense that the Egyptian taskmasters who enslaved the Jewish people would have offered them at no cost good food to eat.  Rather, what it means is that they were free from the obligation to do mitzvos.  

R' Mordechai Eliyahu, whose hilula is coming up, pointed out that you didn't have to take terumos and maasros from mon, you didn't have the shecht mon, you don't do other mitzvos with mon.  The only chiyuv that Bn"Y now had was a chiyuv bracha.  What's the big deal about saying a bracha?

You can only ask that question if you don't think about what a bracha is before saying it.  If you understand that every time you make a bracha you are acknowledging that everything is b'yad Hashem and you are not in control of your life, then yes, it is a big deal.  It's that sense of dependency that Bn"Y found stifling.

R' Levi Yitzchak m'Berdichiv, k'darko, reads the entire complaint in a way that is a zechus for Bn"Y.  The mon could taste like anything you could think of, but obviously, a person can only think of that which he has seen or tasted before.  What could Bn"Y think of?  What foods were they familiar with?  They could think of the fish they had eaten in Egypt, they could think of the vegetables they had eaten, but no one had had ever had cholent before, so how could they think of cholent and turn mon into cholent?!  Or, as the Berdichever puts it, no one in the world had ever tasted meat that was prepared k'halacha, fulfilling the mitzvah of shechita, of melicha, etc.  You can't shecht a fish or a vegetable -- it's  חִנָּ֑ם from mitzvos.  The taam of food that is prepared k'halacha is a different taam, as it is invested with a flavor of ruchniyus.  Bn"Y wanted food with that taam, food that had the flavor of kiyum mitzvos, not  חִנָּ֑ם, and so they asked for real meat so they could shecht and prepare it and add that flavor to their memory banks so they could going forward get it from the mon as well.  (See Sefas Emes for a different limud zechus on the complaint of Bn"Y).