@AuraEnabled
public static Map<String, List<sObject>> returnSObjectMap() {...}

getsobjectMap : function(component) {
var action = component.get("c.returnSObjectMap");
action.setCallback(this, function(a) {
    if (a.getState() === "SUCCESS") {
        var sobjectFields = a.getReturnValue();
        var sobjectFieldsFinal = [];
        for (var key in sobjectFields) {
            sobjectFieldsFinal.push({value:sobjectFields[key], key:key});
        }
        component.set('v.sobjectsMap',sobjectFieldsFinal);
    } else if (a.getState() === "ERROR") {

    }
});

$A.enqueueAction(action);}

...
<aura:attribute name="sobjectsMap" type="List"/>
...
<aura:iteration items="{! v.sobjectsMap }" var="sobjectList" indexVar="key">
    <apex:iteration items="{! sobjectList }" var="sobject">
    ...
    </apex:iteration>
</aura:iteration>

<apex:iteration items="{! sobjectList }" var="sobject">